Time-resolved fluorescence spectroscopy is an indispensable tool in the chemical, physical and biological sciences for the study of fast kinetic processes in the subpicosecond to microsecond time scale. This review focuses on the development and modern implementation of the frequency domain approach to time-resolved fluorescence. Both intensity decay (lifetime) and anisotropy decay (dynamic polarization) will be considered and their application to intrinsic protein fluorescence will be highlighted. In particular we shall discuss the photophysics of the aromatic amino acids, tryptophan, tyrosine and phenylalanine, which are responsible for intrinsic protein fluorescence. This discussion will be illustrated with examples of frequency domain studies on several protein systems.