Comprehensive but user-friendly, reasonably challenging but accessible to a variety of levels, Yoga Shakti is inarguably one of the best yoga products on the market--no small achievement when one considers just how many of them there are. Instructor-creator Shiva Rea, a California-based yogi of considerable international renown, specializes in vinyasa yoga (a vinyasa is essentially a series of flowing, connected poses, incorporating breath and movement and cultivating heat, strength, flexibility, and balance). Four complete practices are presented, ranging in length from 42 to 85 minutes and including Basic Flow (for beginners), two more vigorous Solar Flow sequences, and a Lunar Flow sequence falling somewhere in the middle in terms of degree of difficulty; familiar poses like sun salutations, forward bends, lunges, twists, and inversions are featured, although Rea adds many of her own touches (such as the four "dancing warrior" sections, sun salutation variations designed for intermediate users and focusing on hips, backbends, and so on), and there are also sections devoted to pranayama (breathing exercises) and meditation. It's all good, but the principal innovation of Yoga Shakti is the "yoga matrix," which allows users to customize and program their own workouts, drawing on portions of any or all of the four sequences to create series of various lengths and levels of intricacy (some guidelines are suggested for the less experienced) and thus keep their practices fresh and evolving. One can also do the various workouts with or without the instruction audio track on (the Indian-New Age-flavored music is quite good, and usually unobtrusive), and there's even an entire second disc, containing interviews, additional instruction, travel footage (filming took place primarily in India and the Maldive Islands), and more. All in all, Yoga Shakti is most impressive. --Sam Graham