  This article gives an overview of many of the recent developments in
understanding the structure of relativistic scattering amplitudes in gauge
theories ranging from QCD to N=4 super-Yang-Mills theory, as well as
(super)gravity. I also provide a pedagogical introduction to some of the basic
tools used to organize and illuminate the color and kinematic structure of
amplitudes. This article is an invited review introducing a special issue of
Journal of Physics A devoted to "Scattering Amplitudes in Gauge Theories".
