  I discuss different formulations of the relativistic few-body problem with an
emphasis on how they are related. I first discuss the implications of some of
the differences with non-relativistic quantum mechanics. Then I point out that
the principle of special relativity in quantum mechanics implies that the
quantum theory has a Poincar\'e symmetry, which is realized by a unitary
representation of the Poincar\'e group. This representation can always be
decomposed into direct integrals of irreducible representations and the
different formulations differ only in how these irreducible representations are
realized. I discuss how these representations appear in different formulations
of relativistic quantum mechanics and discuss some applications in each of
these frameworks.
