[eng] The theory of strong interactions, Quantum Chromodynamics (QCD), is a gauge theory describing the interactions between quarks, the building blocks of the proton and the neutron, and the gluons. Significant progress has been recently achieved in the computation of scattering amplitudes in gauge theories, particularly for amplitudes with a large number of external legs, both at the leading order and beyond. New approaches which extend the traditional Feynman diagram-based techniques, have been introduced that greatly reduce the complexity of the calculations and often unveil a remarkable simplicity in the results.
I analyze, extend and apply these cutting-edge techniques to the computation of tree and loop-level scattering amplitudes in gauge theories in general and in QCD and N=4 SYM in particular. Several new results are obtained whose relevance is both practical and theoretical. Firstly, they can be used to make more accurate predictions for processes at hadron collider experiments, such as at the Large Hadron Collider, and secondly they pave new ways towards a deeper theoretical understanding of gauge theories in general.