Gauge Theory Amplitudes, Scalar Graphs and Twistor Space

We discuss a remarkable new approach initiated by Cachazo, Svrcek and Witten for calculating gauge theory amplitudes. The formalism amounts to an effective scalar perturbation theory which in many cases offers a much simpler alternative to the usual Feynman diagrams for deriving $n$-point amplitudes in gauge theory. At tree level the formalism works in a generic gauge theory, with or without supersymmetry, and for a finite number of colours. There is also a growing evidence that the formalism works for loop amplitudes.