  Laser cooling and trapping are central to modern atomic physics. The
workhorse technique in cold-atom physics is the magneto-optical trap (MOT),
which combines laser cooling with a restoring force from radiation pressure.
For a variety of atomic species, MOTs can capture and cool large numbers of
particles to ultracold temperatures (<1 mK); this has enabled the study of a
wide range of phenomena from optical clocks to ultracold collisions whilst also
serving as the ubiquitous starting point for further cooling into the regime of
quantum degeneracy. Magneto-optical trapping of molecules could provide a
similarly powerful starting point for the study and manipulation of ultracold
molecular gases. Here, we demonstrate three-dimensional magneto-optical
trapping of a diatomic molecule, strontium monofluoride (SrF), at a temperature
of approximately 2.5 mK. This method is expected to be viable for a significant
number of diatomic species. Such chemical diversity is desired for the wide
array of existing and proposed experiments which employ molecules for
applications ranging from precision measurement, to quantum simulation and
quantum information, to ultracold chemistry.
