The identity of dark matter is a question of central importance in both astro-physics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief sum-mary of the standard model of particle physics and its outstanding problems. We then discuss several dark matter candidates motivated by these problems, includ-ing WIMPs, superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, we critically examine the particle physics moti-vations and pre...