  We discuss some aspects of the relation between space-time properties of
branes in string theory, and the gauge theory on their worldvolume, for models
invariant under four supercharges in three and four dimensions. We show that a
simple set of rules governing brane dynamics reproduces many features of gauge
theory. We study theories with $U(N_c)$, $SO(N_c)$ and $Sp(N_c)$ gauge groups
and matter in the fundamental and two index tensor representations, and use the
brane description to establish Seiberg's electric-magnetic duality for these
models.
