This chapter describes time-resolved spectroscopy which is one of the most powerful tools to investigate the dynamical processes of excited species in semiconductors. The main dynamical processes which can be studied by time-resolved spectroscopy are, 1) the dephasing, 2) the intra-band and intersubband relaxation processes, and 3) the interband recombination- or lifetime. Equipment for time-resolved spectroscopy are described. To obtain time resolution, a pulsed or temporally modulated excitation source and possibly a time-resolved detection system are needed. The operation of a streak camera and a set-up for an autocorrelation measurement and for time resolved luminescence spectroscopy using a Kerr-cell shutter or up-conversion are illustrated. Luminescence decay measurements are often used to determine the lifetime of some excited species like excitons. The intraband and intersubband-relaxation can be followed most conveniently by time-resolved luminescence spectroscopy. Experimentally more difficult is to monitor the temporal evolution of a cloud of carriers created with some excess energy in time-resolved pump-and-probe spectroscopy or differential transmission spectroscopy. Some examples of coherent processes are discussed. Four-wave mixing spectroscopy in the two-beam, self-diffraction configuration and using two short incident pulses is the usual way to determine the dephasing time of a resonance in a quantum well or -wire or -dot sample in the time domain. In a homogeneously broadened transition all oscillators have identical eigenfrequency. If the resonance is inhomogeneously broadened, the polarization of every individual oscillator decays slowly (dashed lines), but their superposition, i.e. the macroscopic polarization, decays faster, due to the increasingly destructive interference of the oscillators.