  The dark matter distribution on small scales may depend on the the properties
of the first generation of dark matter halos to form, which is in turn
determined by the microphysics of the dark matter particles. We overview the
microphysics of WIMPs and our calculations of the collisional damping and free
streaming scales. We then plot the resulting density perturbation power
spectrum and red-shift at which typical halos form, taking into account the
effect of uncertainties in the WIMP properties (mass and interaction channel)
and the primordial power spectrum. Finally we review recent developments
regarding the properties and fate of the first WIMPy halos.
