Yoga as therapy is the use of yoga as exercise, consisting mainly of postures called asanas, as a gentle form of exercise and relaxation applied specifically with the intention of improving health. This form of yoga is widely practised in classes, and may involve meditation, imagery, breath work (pranayama) and calming music as well as postural yoga. At least three types of health claim have been made for yoga: magical claims for medieval haṭha yoga, including the power of healing; unsupported claims of benefits to organ systems from the practice of asanas; and more or less well supported claims of specific medical and psychological benefits from studies of differing sizes using a wide variety of methodologies. Systematic reviews have found beneficial effects of yoga on low back pain and depression, but despite much investigation little or no evidence for benefit for specific medical conditions. Study of trauma-sensitive yoga has been hampered by weak methodology. Context Yoga classes used as therapy usually consist of asanas (postures used for stretching), pranayama (breathing exercises), and relaxation in savasana (lying down). The physical asanas of modern yoga are related to medieval haṭha yoga tradition, but they were not widely practiced in India before the early 20th century. The number of schools and styles of yoga in the Western world has grown rapidly from the late 20th century. By 2012, there were at least 19 widespread styles from Ashtanga Vinyasa Yoga to Viniyoga. These emphasise different aspects including aerobic exercise, precision in the asanas, and spirituality in the haṭha yoga tradition. These aspects can be illustrated by schools with distinctive styles. Thus, Bikram Yoga has an aerobic exercise style with rooms heated to and a fixed sequence of 2 breathing exercises and 26 asanas performed in every session. Iyengar Yoga emphasises correct alignment in the postures, working slowly, if necessary with props, and ending with relaxation. Sivananda Yoga focuses more on spiritual practice, with 12 basic poses, chanting in Sanskrit, pranayama breathing exercises, meditation, and relaxation in each class, and importance is placed on vegetarian diet. Types of claim At least three different types of claim of therapeutic benefit have been made for yoga from medieval times onwards, not counting the more general claims of good health made throughout this period: magical powers, biomedical claims for marketing purposes, and specific medical claims. Neither of the first two are supported by reliable evidence. The medical claims