The gain spectra and the dynamics of semiconductor lasers are investigated on timescales from continuous wave down to the 100 fs regime. First, we measure continuous wave gain spectra over a wide spectral range with a transmission experiment using the ultrabroad spectrum of a 10 fs Ti:sapphire laser. The agreement between our experimental results and a parameter-free microscopic theory is excellent, only if carrier scattering and polarization are treated correctly. Accordingly, carrier thermalization and material polarization dynamics have to be considered in order to achieve a fundamental understanding of the dynamics of semiconductor lasers. The ultrafast carrier and polarization dynamics are experimentally investigated by femtosecond heterodyne pump-probe and four-wave-mixing experiments. Finally, we study the influence of carrier thermalization on the emission dynamics of a vertical-cavity surface-emitting laser (VCSEL). The VCSEL is optically pumped with femtosecond pulses from a Ti:sapphire laser and the temporal evolution of its emission is investigated as a function of the initial excess energy of the photoexcited carriers. The dynamics become slower for larger excess energies since carrier cooling is slower. The fastest response is obtained for resonant pumping.