  We report magneto-optical trapping (MOT) of a polyatomic molecule, calcium
monohydroxide (CaOH). The MOT contains $2.0(5)\times 10^4$ CaOH molecules at a
peak density of $3.0(8)\times10^{6}$ cm$^{-3}$. CaOH molecules are further
sub-Doppler laser cooled in an optical molasses, to a temperature of 110(4)
$\mu$K. The temperatures and densities achieved here make CaOH a viable
candidate for a wide variety of quantum science applications, including the
creation of optical tweezer arrays of CaOH molecules. This work also suggests
that laser cooling and magneto-optical trapping of many other polyatomic
species will be both feasible and practical.
