How can one best present a fly to a trout so that the imitation appears credible? Until the publication of this book, it was mostly speculation about how the fish perceives the offering. The authors then explored, using an underwater camera, how fish can see insects, lines, and lures. The results are summarized here. Additionally, the reader receives precise instructions on how to catch trout in deep, fast-flowing water and how the angler can best approach the desired catch. Tips on tying realistic flies complete this work, which has been essential reading for every fly fisherman since its first publication.