अथ योगानुशासनमः
योगश्चित्तवृत्तिनिरोधः
तदा द्रष्टुः स्वरूपेऽवस्थानमः

atha yogānuśāsanamḥ
yogaścittavṛttinirodhaḥ
tadā draṣṭuḥ svarūpe'vasthānamḥ

Now then yoga is explained
Yoga is the cessation of fluctuations of the mind
Then the seer abides in the Self

Thus begins the Yoga Sutra.  Composed around 200 B.C.E. by Patanjali, it is one of the earliest and most important treatises on the practice and philosophy of yoga. 

In the Yoga Sutra, Patanjali defines the "eight means of attaining yoga," which are collectively referred to as Ashtanga (ashtanga is a Sanskrit word meaning "eight limbs").  The modern practice known as Ashtanga Yoga is a system integrating these eight disciplines, of which the physiological aspects include breath, movement and posture.  The system was codified and published by Sri K. Pattabhi Jois following training under his guru Sri T. Krishnamacharya.  Pattabhi Jois went on to found the Ashtanga Yoga Research Institute in Mysore, India, and taught there until his death.  The training there is offered as an authentic realization of Patanjali's system of yoga, and continues under the direction of Sharath Rangaswamy.

My initial interest in the practice was stirred by Rebecca Mead's profile of Pattabhi Jois and the Ashtanga community in Mysore.  By coincidence, at that time I was performing concerts with kirtan singer Seán Johnson, who is also a yoga instructor and was offering an introductory course in Ashtanga yoga.  I've been practicing more or less continuously ever since.