The authors' aim in this book is to give a pedagogical introduction to the helicity method, illustrated by many examples, and to summarize some ten years of research on the subject. Only some degree of familiarity with Feynman diagrams and Dirac algebra is required from the reader. The book also provides an extensive list of helicity amplitudes and cross-sections for many of the most important QED and QCD processes at high energies, some being new and not available in the literature previously.