  We study dynamical supersymmetry breaking in four dimensions using the
fivebrane of M theory, in particular for the Izawa-Yanagida-Intriligator-Thomas
(IYIT) model, which we realize as the worldvolume theory of a certain M-theory
fivebrane configuration. From the brane point of view, supersymmetry is broken
when a holomorphic configuration with the proper boundary conditions does not
exist. We discuss the difference between explicit and spontaneous supersymmetry
breaking and between runaway behavior and having a stable vacuum. As a
preparation for the study of the IYIT model, we examine a realization of the
orientifold four-plane in M theory. We derive known as well as new results on
the moduli spaces of N=2 and N=1 theories with symplectic gauge groups. These
results are based on a hypothesis that a certain intersection of the fivebrane
and the Z_2 fixed plane breaks supersymmetry. In the IYIT model, we show that
the brane exhibits runaway behavior when the flavor group is gauged. On the
other hand, if the flavor group is not gauged, we find that the brane does not
run away. We suggest that a stable supersymmetry-breaking vacuum is realized in
the region beyond the reach of the supergravity approximation.
