  This thesis aims at providing better understanding of the perturbative
expansion of gauge theories with and without supersymmetry. At tree level, the
BCFW recursion relations are analyzed with respect to their validity for
general off-shell objects in Yang-Mills theory, which is a significant step
away from their established zone of applicability. Unphysical poles constitute
a new potential problem in addition to the boundary behavior issue, common to
the on-shell case as well. For an infinite family of massive fermion currents,
both obstacles are shown to be avoided under the certain conditions, which
provides a natural recursion relation. At one loop, scattering amplitudes can
be calculated from unitarity cuts through their expansion into known scalar
integrals with free coefficients. A powerful method to obtain these
coefficients, namely spinor integration, is discussed and rederived in a
somewhat novel form. It is then used to compute analytically the infinite
series of one-loop gluon amplitudes in N = 1 super-Yang-Mills theory with
exactly three negative helicities. The final part of this thesis concerns the
intriguing relationship between gluon and graviton scattering amplitudes, which
involves a beautiful duality between the color and kinematic content of gauge
theories. This BCJ duality is extended to include particles in the fundamental
representation of the gauge group, which is shown to relieve the restriction of
the BCJ construction to factorizable gravities and thus give access to
amplitudes in generic (super-)gravity theories.
