  We present a new paradigm for achieving thermal relic dark matter. The
mechanism arises when a nearly secluded dark sector is thermalized with the
Standard Model after reheating. The freezeout process is a number-changing 3->2
annihilation of strongly-interacting-massive-particles (SIMPs) in the dark
sector, and points to sub-GeV dark matter. The couplings to the visible sector,
necessary for maintaining thermal equilibrium with the Standard Model, imply
measurable signals that will allow coverage of a significant part of the
parameter space with future indirect- and direct-detection experiments and via
direct production of dark matter at colliders. Moreover, 3->2 annihilations
typically predict sizable 2->2 self-interactions which naturally address the
`core vs. cusp' and `too-big-to-fail' small structure problems.
