We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is ≊50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in ≊50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.