  This article provides an introduction to on-shell recursion relations for
calculations of tree-level amplitudes. Starting with the basics, such as spinor
notations and color decompositions, we expose analytic properties of
gauge-boson amplitudes, BCFW-deformations, the large $z$-behavior of
amplitudes, and on-shell recursion relations of gluons. We discuss further
developments of on-shell recursion relations, including generalization to other
quantum field theories, supersymmetric theories in particular, recursion
relations for off-shell currents, recursion relation with nonzero boundary
contributions, bonus relations, relations for rational parts of one-loop
amplitudes, recursion relations in 3D and a proof of CSW rules. Finally, we
present samples of applications, including solutions of split helicity
amplitudes and of N= 4 SYM theories, consequences of consistent conditions
under recursion relation, Kleiss-Kuijf (KK) and Bern-Carrasco-Johansson (BCJ)
relations for color-ordered gluon tree amplitudes, Kawai-Lewellen-Tye (KLT)
relations.
