Nymphs -- the underwater stages of the aquatic insects -- comprise ninety percent of the trout's diet. Knowing how to identify nymphs, tie successful imitations, and fish them correctly are essential fly-fishing skills. This book presents a basic approach to all these techniques in one comprehensive lesson, covering both the traditional and new theories about fishing the underwater fly -- new casts, finding lies, detecting the strike, fishing depth, tackle, proper presentation and much more valuable information, fully illustrated with line drawings.