  Lecture notes on Poincar\'e-invariant scattering amplitudes and tree-level
recursion relations in spinor-helicity formalism. We illustrate the
non-perturbative constraints imposed over on-shell amplitudes by the Lorentz
Little Group, and review how they completely fix the three-point amplitude
involving either massless or massive particles. Then we present an introduction
to tree-level BCFW recursion relations, and some applications for massless
scattering, where the derived three-point amplitudes are employed.
