Yoga is a spiritual tradition that began in India about 5,000 years ago. Historically its practices have been adopted by such religions as Hinduism, Jainism, and Buddhism. However, the practice of yoga is compatible with any religion, as well as atheism.
The word yoga means union in Sanskrit. Many of the yoga traditions believe that the practices of yoga lead to union of a person with his or her true nature, or with God or the universe (however one wishes to conceptualize it). How this is interpreted and the means employed to get there differ in the various yoga traditions.
Patanjali, author of the classic yoga text, the Yoga Sutra, defined yoga as the quieting of the fluctuations of the mind. From a yogic perspective, the mind, and its ceaseless internal monologue, is the source of suffering. The various yoga practices serve to calm the mind. From a wellspring of relaxed concentration-the hallmark of yoga-comes inner peace, heightened creativity, and awareness of our essential nature.
When people in the West hear the word yoga, they usually think of the stretching and strengthening poses known as asana [AHS ah nah], which are the primary focus of most yoga classes. But asana is only one aspect of a multidimensional system that includes:
There appear to be numerous benefits from the practice of yoga.
Arguably the most comprehensive method of stress-reduction  ever developed, yoga offers numerous tools to shift the balance of the autonomic nervous system to the parasympathetic side, calm the mind, and lower the levels of stress hormones such as cortisol.
This is likely to be of benefit not just for those suffering from burnout and such stress-related conditions as insomnia and irritable bowel syndrome , but also in the treatment and prevention of leading causes of death, including heart disease , hypertension, and diabetes , all of which may be adversely affected by high levels of stress .
Beyond stress reduction, the practice of yoga has other health benefits, including increased strength and flexibility, better balance and coordination, improved reaction times, better lung function, heightened cardiovascular conditioning, and weight loss.
Psychological benefits of practice include relaxation, greater equanimity, better concentration, and improved mood.
There is substantial overlap between yoga practices and other mind-body therapies . Biofeedback  techniques, in which patients are taught to tune into sensations in their body and regulate their breathing in order to facilitate relaxation of muscles, relieve stress or warm their hands and feet, are based on yogic principles.
In addition to mind-body therapies, yoga practices have been adapted in manual therapies. A number of physical exercises used in rehabilitation from injuries are based on yoga asana (postures). One example is the gentle backbends often used after back injuries.
There has been an increased amount of research (particularly randomized controlled trials) on yoga's effects throughout the past few decades. Types of medical conditions have included psychopathological (e.g. depression and anxiety ), cardiovascular (e.g. hypertension, heart disease ), respiratory (e.g. asthma), diabetes , and others.
Most of this research comes from India, although there is increasing contribution from researchers in the U.S. and England. Not all of this research is of high quality methodologically, but in recent years the quality has generally improved. Although standardized protocols have been developed to meet the needs of scientific study, most yoga, and particularly yoga therapy, is personalized to the individual and is not strictly based on the Western medical diagnosis.
In addition, different techniques have different effects on each individual and even on one individual at different times or stages in his or her practice. Thus, what is typically measured in scientific studies of yoga doesn't reflect what usually happens in the real world. Yogis would also argue that the one-size-fits-all approaches used in clinical studies are likely to be much less effective than a personal yoga practice.
In addition, in both India and the West, yoga is used therapeutically for many conditions based largely on experiential evidence. In North America, workshops are offered on hatha yoga for people with such conditions as HIV/AIDS, scoliosis, back pain , depression , pregnancy , and cancer .
There are no universal standards or accreditation for either yoga teachers or yoga therapists. However, the Yoga Alliance is a national non-profit organization that certifies teachers from different traditions at either the 200- or 500-hour level of training.
Currently no states require licensure or certification for yoga teachers or therapists, although that may change in coming years.
The training and certification of yoga teachers will vary greatly. It is helpful to ask prospective teachers to describe the following:
Anusara yoga 
Ashtanga yoga 
Iyengar yoga 
Kundalini yoga 
Yoga Alliance