The kinetic decoupling of weakly interacting massive particles (WIMPs) in the early universe sets a scale that can directly be translated into a small-scale cutoff in the spectrum of matter density fluctuations. I present a formalism which allows to describe this decoupling process in great detail and to determine the cutoff scale to a high accuracy; with decoupling temperatures of several MeV to a few GeV, depending on the details of the underlying WIMP microphysics, the smallest protohalos to

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... be formed range between 10 −11 and almost 10 −3 solar masses. Observational consequences and prospects to probe this small-scale cutoff, which would provide a fascinating new window into the particle nature of dark matter, are briefly discussed.