High-amplitude over-densities in the early Universe are expected to gravitationally collapse shortly after matter-radiation equality, triggering the formation of ultracompact minihalos of dark matter (UCMHs).  Because the mass of a UCMH is directly linked to the scale of the density perturbation from which it formed, UCMHs turn out to be very clean (and sensitive) probes of early-time, small-scale cosmology.  I will describe our current understanding of UCMH structure and formation, the present and forthcoming limits on their abundance from gamma-rays, pulsar timing and astrometric microlensing, and the resulting implications for the primordial power spectrum of perturbations, non-Gaussianities, cosmic strings and inflation.