The regulation of Prāṇa. Prāṇa is the individual life-force, which manifests as the breath in the physical. It includes both regulation and control and affects both the body and mind. Many consider it the most important aspect of yoga.

Why Do Prāṇāyāma? Defines and describes the whys for doing prāṇāyāma.
Your Best Breath Learn the difference between diaphragmatic and chest breathing.
Prāṇāyāma for Hatha and Ashtanga Yoga Provides description of types of prāṇāyāma, more involved practices and mudras to use during practice.
Yoga and Prāṇāyāma Define what Yogic breathing with a description of prāṇa in the general sense.
The Breathing Project A New York City studio dedicated to teaching the individualized, breath-centered principles of Yoga in the tradition of Desikachar and Krishnamacharya.
Give Breathing A Chance Max Strom encourages us to embrace pranayama as we do asana. This article explains how to perform five basic prāṇāyāma practices. Please note is always best to learn these with a teacher to be sure you are applying the technique properly.
The Science of Breathing Learn about the cycle of air and how it moves within the body.

Videos
See B.K.S. Iyengar speak on prāṇāyāma.
Books, CDs, etc.
- Light on Prāṇāyāma by B.K.S. Iyengar
In this classic yoga best-seller a world-renowned yoga master shares the techniques of breathing together with a comprehensive background of yoga philosophy.
- Prāṇāyāma Beyond the Fundamentals by Richard Rosen
For serious students of yoga, who have an established prāṇāyāma practice. Traditional techinques for deepening prāṇāyāma practice.