Buddhist monasticism is one of the earliest surviving forms of organized monasticism in the history of religion. It is also one of the most fundamental institutions of Buddhism. Monks and nuns are considered to be responsible for the preservation and dissemination of the Buddha's teaching and the guidance of Buddhist lay people.
The order of Buddhist monks and nuns was founded by Gautama Buddha during his lifetime between the fifth and fourth centuries BC. The Buddhist monastic lifestyle grew out of the lifestyle of earlier sects of wandering ascetics, some of whom the Buddha had studied under. It was not really isolationist or eremetic: the sangha was dependent on the lay community for basic provisions of food and clothing, and in return sangha members helped guide lay followers on the path of Dharma. Individuals or small groups of monks – a teacher and his students, or several monks who were friends – traveled together, living on the outskirts of local communities and practicing meditation in the forests. Monks and nuns were expected to live with a minimum of possessions, which were to be voluntarily provided by the lay community. Lay followers also provided the daily food that monks required, and provided shelter for monks when they were needed. Some Buddhist schools assert that during the Buddha's time, many retreats and gardens were donated by wealthy citizens for monks and nuns to stay in during the rainy season (although there is as yet no archaeological evidence to support this claim - evidence only exists for such monastic enclosures at a much later date). Out of this tradition grew two kinds of living arrangements for monastics, as detailed in the Mahavagga section of the Vinaya and Varsavastu texts:
One of the more famous Arama is Anathapindika's, known as Anathapindikassa arame, built on Prince Jeta's grove. It had buildings worth 1.8 million gold pieces built in a beautiful grove, with the total gift worth 5.4 million gold pieces.
After the parinirvana of the Buddha, the Buddhist monastic order developed into a primarily cenobitic movement. The practice of living communally during the rainy vassa season, prescribed by the Buddha, gradually grew to encompass a settled monastic life centered on life in a community of practitioners. Most of the modern disciplinary rules followed by monks and nuns—the Patimokkha—relate to such an existing, prescribing in great detail proper methods for living and relating in a community of monks or nuns. The number of rules observed varies with the order; Theravada monks follow around 227 rules. There are a larger number of rules specified for bhikkhunis.
Buddhism has no central authority, and therefore many different varieties of practice and philosophy have developed over its history, including among monastic communities, sometimes leading to schisms in the sangha. The information presented here, unless otherwise noted, characterises only certain Buddhist monks who follow the most strict regulations of the 'Southern Schools' tradition. The oldest existing set of texts concerning a Buddhist form of life are those of the Pāli Canon. Although no copy of these texts comes from the time of the Buddha, because of its relative age the Pāli Canon is used by some monastic communities to define their conduct and identity. In some schools of Buddhism, notably those lineages in South East Asia (Sri Lanka, Thailand, etc.) that compose Theravada, the Buddhist monastic community is theoretically divided into two assemblies, the male bhikkhu (Pali, Skt. bhikshu) assembly, and the female bhikkhuni (Skt. bhikshuni) assembly. According to some stories, although his followers initially consisted only of men, the Buddha recognized women as followers after his stepmother, Mahaprajapati, asked for and received permission to live as an ordained practitioner. Also the Buddha's disciple Ananda strongly insisted on including female order. Female monastic communities in the bhikkhuni lineage were never established in the Vajrayana communities of Tibet and Nepal; Theravada communities formerly existed, but died out between the 11th and 14th century. Ordination in the bhikkhuni lineage continues to exist among East Asian communities, and attempts have been made at a revival in Southeast Asia and Sri Lanka. Such divisions are more rarely made in the Northern schools, or in the West.
Monks and nuns are expected to fulfill a variety of roles in the Buddhist community. First and foremost, they are expected to preserve the doctrine and discipline now known as Buddhism. They are also expected to provide a living example for the laity, and to serve as a "field of merit" for lay followers, providing laymen and women with the opportunity to earn merit by giving gifts and support to the monks. In return for the support of the laity, monks and nuns are expected to live an austere life focused on the study of Buddhist doctrine, the practice of meditation, and the observance of good moral character. The relative degree of emphasis on meditation or study has often been debated in the Buddhist community. Many continued to keep a relationship with their original families.
A Bhikkhu or Bhikkhuni first ordains as a Samanera (novice) for a year or more. There are some conditions which must be met in order to be allowed into Buddhist monaticism, such as age between 7 and 70 and haven't broken sīla in some manners when undertaking them. Male novices often ordain at a very young age, but generally no younger than 8. Women usually choose to ordain as adults, since there is no expectation that they do so in childhood. Samaneras live according to the Ten Precepts, but are not responsible for living by the full set of monastic rules, vinaya. Higher ordination, conferring the status of a full Bhikkhu or Bhikkhuni, is usually given only to those 20 or older. Women monastics follow a similar progression, but are required to live as Samaneras for a longer period of time, typically five years. In Thailand, monks are considered well supported by the community, and in return the monks offer guidance in life and in the Dharma. They can give the Bhikkhu or Bhikkhuni vows back and return to non-monastic living. According to the vinaya, the sangha should not accept a former Bhikkhuni to retake these vows, and for Bhikkhu up to taking three or seven times in a life. Breaking some important vinaya in manners according to the Vinaya Pitaka would not be accepted for monasticism again for a lifetime. In Burma, Taiwan and Hong Kong, there is sometimes short term monastic ordination available to lay buddhists to take some vows for a week up to month. In Thailand such arrangements are currently available for male only.
The disciplinary regulations for monks and nuns are intended to create a life that is simple and focused, rather than one of deprivation or severe asceticism. Celibacy is of primary importance in monastic discipline, seen as being the preeminent factor in separating the life of a monastic from that of a householder. Depending on the tradition and the strictness of observation, monastics may eat only one meal a day, provided either by direct donations of food from lay supporters, or from a monastery kitchen that is stocked (and possibly staffed) by lay supporters.
Unlike Christian monastics, some schools of Buddhist monastics are not required to live a life of obedience to a superior. However, it is expected that monastics will offer respect to senior members of the Sangha (in Thai tradition, seniority is based on the number of rains retreats, vassas, that one has been ordained). The Buddha did not appoint a successor, nor did he specify rules mandating obedience in the monastic code. Individual groups of monastics are expected to make decisions collectively through regular gatherings of the community, at which decisions regarding violations of monastic rules and the dispositions of communal property are to be made. Individual relationships of teacher/student, senior/junior, and preceptor/trainee may be observed among groups of monastics, but there are no formal positions, nor is there any authority to give orders or commands invested in senior monks. An abbess or abbot, typically a senior monastic still young enough to be active, is usually responsible for the day-to-day administration of the monastery, and may appoint others to assist with the work. In some traditions, the abbess/abbot is chosen by a vote of the monastics in a monastery. In other traditions (Thailand, for example), the abbot is chosen by the lay community.
Monastic practices vary significantly according to location. In part, this can be attributed to differences in the scriptural and doctrinal traditions that were received in different parts of the Buddhist world. Additionally, local concessions to social, geographical, and climatic conditions have been adopted by most monastic orders in order to smooth the integration of monks into local communities, and to ensure that monks live in a safe and reasonable manner. In cold climates, for instance, monks are permitted to own and wear additional clothing not specified in the scriptures. In areas where begging rounds are impossible (due to traffic, geography, or disfavor by the lay community), monks more commonly employ a kitchen staff of monks or lay followers who are responsible for providing meals for the community.
In Tibet, before the Chinese invasion in the late 1940s and early 1950s, more than half of the country's male population was ordained. Today, this is no longer the case. While generally adhering to a Mahayana tradition that advocates the virtues of vegetarianism, Tibetan monks generally eat meat as a concession to climatic conditions that make a plant-based diet largely unfeasible. Tibetan monks follow the Mūlasarvāstivāda vinaya lineage.
Lamas who take bhikṣu vows are not allowed to marry. The Nyingma school includes a mixture of bhikṣus and non-celibate ngakpas, and it is not unusual for lamas to wear robes closely resembling monastic garb despite them not being bhikṣus. Sakya school does not allow monks to get close to women after they have sons. Gelug school emphasized Vinaya ethics and monastic discipline; Choekyi Gyaltsen refused to wear monk clothing after he married. Kagyu monks are also required to return to non-monastic life to marry.
In East Asia, monastics live in greater isolation from the lay population than is observed in most Theravada countries. Because of local conditions of geography and climate, as well as local attitudes towards begging, monks generally do not make begging rounds in China, Korea, Vietnam, and many parts of Japan. Instead, monasteries receive donations of bulk food (such as rice) and funds for the purchase of food that is then stored and prepared at the monastery. Many monks and nuns are vegetarians and, after Baizhang Huaihai, many monks farm food to eat; some work or sell. Most eat after noon. The management of the kitchen and monastery properties may be the purview of a specially designated layman or a monk who has been given a special role by the abbot of the monastery. Monks chant many mantras in regular living. Buddhist monks and nuns lived together in China in Lingshansi (河南信阳灵山寺), Luming'an (河南固始九华山妙高寺鹿鸣庵), Hong'ensi (重庆鸿恩寺), Ciyunsi (重庆慈云寺), Sandingsi (西藏山南桑丁寺), Chahuasi (云南茶花寺)
Monastics in Japan are particularly exceptional in the Buddhist tradition because the monks and nuns can marry after receiving their higher ordination. This idea is said to be introduced by Saichō, the founder of the Tendai school, who preferred ordaining monks under the Bodhisattva vows rather than the traditional Vinaya. There had long been many instances of Jōdo Shinshū priests and priestesses marrying, influenced by the sect's founder Shinran, but it was not predominant until a government Nikujiku Saitai Law (肉食妻帯) was passed during the Meiji Restoration that monks or priests of any Buddhist sect are free to seek wives. This practice influenced Korea and Taiwan. A nun in Taiwan gave birth. Some Korean monks live with wives in their monasteries.
Monks of certain Chinese Buddhist sects are allowed to marry,  such as in historical Yunnan, Lingnan and Taiwan. Buddhism in China was historically viewed to be the lowest of the three major religions in China because buddhist monks came from the lower classes and were both poor and uneducated.
In Sri Lanka, Thailand, Cambodia, Laos, and Myanmar, where the Theravada school is dominant, there is a long tradition of temporary ordination. During a school break, many young men usually ordain for a week or two to earn merit for loved ones and to gain knowledge of Buddhist teachings. In most countries, this temporary ordination occurs during the vassa retreat, which is regarded as a period of intensified spiritual effort by local Buddhists. Men in Thailand typically ordain only before being married; men in Laos and Myanmar could traditionally return to the monastery from time to time after being married, provided that they secured their wives' permission. Theravada monks are also most likely to engage in traditional practices of collecting alms, although the urbanization of parts of Southeast Asian (particularly Thailand) has presented a challenge to this practice.
In Thailand, where the Buddhist institution has traditionally been closely associated with the government and the institution of kingship, a more hierarchical structure has evolved to deal with the administration and regulation of monasteries. This system initially stemmed from a system of royal patronage, in which monks who were appointed the abbots of 'royal monasteries' (those endowed and supported by members of the royal family) were accorded greater respect than those who headed more conventional monasteries. This system remained fairly unstructured until the modernization efforts of the 19th century, during which a more formal system of governance was created by the central government. Modern Thai monks are ranked according to their ability to pass examinations in Buddhist doctrine and the Pali language, and are appointed to successively higher positions in the ecclesiastic hierarchy on the basis of these exams, as well as their support among influential members of the royal family and government. Local affairs continue to be handled primarily by the local monastic and lay community, but nationwide efforts (such as curriculum decisions for monastic schools, and the authoritative form of scriptures and rituals) are typically made by the central hierarchy.