  In the cold dark matter scenario, dark matter halos are assembled
hierarchically from smaller subunits. Some of these subunits are disrupted
during the merging process, whereas others survive temporarily in the form of
subhalos. A long-standing problem with this picture is that the number of
subhalos predicted by simulations exceeds the number of luminous dwarf galaxies
seen in the the vicinity of large galaxies like the Milky Way. Many of the
subhalos must therefore have remained dark or very faint. If cold dark matter
subhalos are as common as predicted, gravitational lensing may in principle
offer a promising route to detection. In this review, we describe the many ways
through which lensing by subhalos can manifest itself, and summarize the
results from current efforts to constrain the properties of cold dark matter
subhalos using such effects.
