The bulk of the matter in the present universe is dark. The most attractive possibility for the nature of the dark matter is a new species of elementary particle known as a WIMP (a Weakly Interacting Massive Particle). After a discussion of how a WIMP might fit into models of particle physics, I will review the current situation with respect to direct detection, indirect detection, and collider production of WIMPs. Rapid advances in the field should enable us to answer by the end of the decade whether our universe is dominated by WIMPs.