  Many examples of low-energy dualities have been found in supersymmetric gauge
theories with four supercharges, both in four and in three space-time
dimensions. In these dualities, two theories that are different at high
energies have the same low-energy limit. In this paper we clarify the relation
between the dualities in four and in three dimensions. We show that every four
dimensional duality gives rise to a three dimensional duality between theories
that are similar, but not identical, to the dimensional reductions of the four
dimensional dual gauge theories to three dimensions. From these specific three
dimensional dualities one can flow to many other low-energy dualities,
including known three dimensional dualities and many new ones. We discuss in
detail the case of three dimensional SU(N_c) supersymmetric QCD theories,
showing how to derive new duals for these theories from the four dimensional
duality.
