Degenerate four-wave mixing (DFWM), using similar to 60 femtosecond (fs) laser pulses, is introduced to study transition-state dynamics of chemical reactions in the gas phase. The ultrafast techniques are applied to a range of systems, atomic, unimolecular, and bimolecular. It is shown how fs DFWM can be incorporated in different temporal pulse schemes to extract the dynamics. The DFWM beams are configured in a folded boxcar geometry, producing a spatially separated, background-free, femtosecond signal pulse. Aspects of the technique, such as absorption, are investigated. We have taken advantage of the relatively broad spectral width of the fs pulses and extended the techniques to two-color grating experiments in the gas phase. The unimolecular system, NaI, provided a means of testing this new approach. Our experimental observations of the wave packet motion are in excellent agreement with results obtained using laser-induced fluorescence (LIF). A control experiment was also performed on this system, demonstrating the advantages of the nonlinear technique. Bimolecular reactions were initiated for the system Na+H-2. Atomic sodium was investigated with fs DFWM and the oscillatory wave packet behavior (2 ps period) was observed, corresponding to the fine structure splitting of the 3p level (17.2 cm(-1)). We also explored the application of fs DEWM to the reactive and nonreactive collisions of the Na+H-2 system, which serves as a good model for studying dynamics of nonadiabatic quenching processes and collision complexes.