  We review the structure of gauge theory scattering amplitudes at tree level
and describe how a compact expression can be found which encodes all the
tree-level amplitudes in the maximally supersymmetric N=4 theory. The
expressions for the amplitudes reveal a dual superconformal symmetry. We
describe how these ideas can be extended to leading singularities and the loop
integrand in the planar theory and discuss the appearance of dual conformal
symmetry in higher-dimensional gauge theories. This article is an invited
review for a special issue of Journal of Physics A devoted to "Scattering
Amplitudes in Gauge Theories".
