Numerical and experimental studies of the propagation of femtosecond-durati
on optical pulses in quadratic nonlinear media are presented. Pulse evoluti
on is investigated over wide ranges of initial intensity and phase mismatch
between fundamental and harmonic waves, and the conditions that produce tw
o-dimensional spatiotemporal solitons are delineated. Spatiotemporal solito
ns can be generated when the group velocities of the fundamental and harmon
ic fields are quite different, for proper choice of the phase mismatch. The
factors that limit the formation of spatiotemporal solitons are discussed.