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While yoga's roots are ancient, the modern yoga class is a relatively recent phenomenon, as is the concept of a yoga teacher-training program. But as yoga has become big business: With nearly $17 billion spent on yoga clothing, equipment, classes and accessories in the United States in 2016, the number of yoga teachers, training programs and practice options has skyrocketed.

There are two main organizations that provide credentials for yoga teachers and yoga therapists:

The Yoga Alliance represents 83,000 teachers and 5,500 schools. Founded in 1999, the nonprofit operates a registry that lists its teachers and training programs. The group's most basic instructor credential, RYT, can be earned after completing a 200-hour training at a registered yoga school, or RYS. Teachers can add an "E" for "experienced" to this title (E-RYT) after having two years of experience and at least 1,000 hours of teaching. For more information, visit yogaalliance.org.

The International Association of Yoga Therapists has 5,700 members in more than 50 countries. Yoga therapy typically involves one-on-one instruction geared to empowering an individual to progress toward improved health. Last year, the association established a certification program with the credential IAYT-Certified Yoga Therapist, or C-IAYT, which requires at least 800 hours of training beyond the prerequisite 200-hour yoga teacher training. To learn more, go to iayt.org.

In addition to these broadly based groups, a growing number of specialty programs offer advanced trainings for yoga teachers who want to work with special populations. These include Yoga for Arthritis (arthritis.yoga), Yoga for Cancer and Yoga for Chronic Pain (both at mindfulyogaworks.com/teacher-training) and Yoga for Seniors (yoga4seniors.com). Full disclosure: I am co-director of Yoga for Seniors.

Graduate-level yoga studies are offered at a few colleges, including the Maryland University of Integrative Health, which offers a master of science in yoga therapy, and Loyola Marymount University in California, which offers a master of arts in yoga studies.

Carol Krucoff