  We introduce a formalism for describing four-dimensional scattering
amplitudes for particles of any mass and spin. This naturally extends the
familiar spinor-helicity formalism for massless particles to one where these
variables carry an extra SU(2) little group index for massive particles, with
the amplitudes for spin S particles transforming as symmetric rank 2S tensors.
We systematically characterise all possible three particle amplitudes
compatible with Poincare symmetry. Unitarity, in the form of consistent
factorization, imposes algebraic conditions that can be used to construct all
possible four-particle tree amplitudes. This also gives us a convenient basis
in which to expand all possible four-particle amplitudes in terms of what can
be called "spinning polynomials". Many general results of quantum field theory
follow the analysis of four-particle scattering, ranging from the set of all
possible consistent theories for massless particles, to spin-statistics, and
the Weinberg-Witten theorem. We also find a transparent understanding for why
massive particles of sufficiently high spin can not be "elementary". The Higgs
and Super-Higgs mechanisms are naturally discovered as an infrared unification
of many disparate helicity amplitudes into a smaller number of massive
amplitudes, with a simple understanding for why this can't be extended to
Higgsing for gravitons. We illustrate a number of applications of the formalism
at one-loop, giving few-line computations of the electron (g-2) as well as the
beta function and rational terms in QCD. "Off-shell" observables like
correlation functions and form-factors can be thought of as scattering
amplitudes with external "probe" particles of general mass and spin, so all
these objects--amplitudes, form factors and correlators, can be studied from a
common on-shell perspective.
