The Kuramoto system is a paradigmatic example that illuminates the
mechanisms by which synchronization arises in a large set of heterogeneous
and globally-coupled oscillators. Motivated by neurobiological dynamics, we
study multiple interacting Kuramoto systems in which the connectivity is
specified by a general matrix. Knowledge of this matrix is sufficient to
determine the onset of collective synchronous behavior. We also consider 
the case of time-dependent coupling among the populations.