  The inverse source problem where an unknown source is to be identified from
the knowledge of its radiated wave is studied. The focus is placed on the
effect that multi-frequency data has on establishing uniqueness. In particular,
it is shown that data obtained from finitely many frequencies is not
sufficient. On the other hand, if the frequency varies within a set with an
accumulation point, then the source is determined uniquely, even in the
presence of highly heterogeneous media. In addition, an algorithm for the
reconstruction of the source using multi-frequency data is proposed. The
algorithm, based on a subspace projection method, approximates the minimum-norm
solution given the available multi-frequency measurements. A few numerical
examples are presented.
