  The hydrodynamics of stellar collisions and mergers is discussed in the
context of blue straggler formation. Emphasis is placed on the very important
question of hydrodynamic mixing during the merger process. Recent results of
three-dimensional hydrodynamic calculations suggest that the merger remnants
produced by stellar collisions are typically {\em not\/} well-mixed. However,
comparisons between the observed colors and numbers of blue stragglers in dense
clusters and the predictions of theoretical calculations for their stellar
evolution appear to require that the initial blue-straggler models be close to
chemically homogeneous. The resolution of this apparent conflict is likely to
involve the development of convection or other thermal instabilitites that can
provide efficient mixing during the contraction of the merger remnant to a
thermal equilibrium state on the main sequence.
