EN
We analyze propagation of ultra short light pulses in a transparent, dispersive, nonlinear medium. A general formula for femtosecond wave packet evolution is developed and applied to specific problems. Theoretical and experimental results for wave packet distortion by lenses, wave packet rotation in birefringent media and group velocity matching in sum frequency generation are presented. Numerical results for splitting of femtosecond wave packets in dispersive Kerr media are also presented.