  We classify all massive irreducible representations of super Poincar\'e in
D=10. New Casimir operators of super Poincar\'e are presented whose eigenvalues
completely specify the representation. It is shown that a scalar superfield
contains three irreducible representations of massive supersymmetry and we find
the corresponding superprojectors. We apply these new tools to the quantization
of the massive superparticle and we show that it must be formulated in terms of
a superfield $B_\mn$ satisfying an adequate covariant restriction.
