We used fiber-optic phase-modulation methods as well as time-correlated single-photon counting to characterize time-dependent migration of 580-740 nm photons in living tissue, in particular in human fingers, forearms, calfs and foreheads. The phase-modulation measurements were extended to 4.2 GHz using an internally cross-correlated gatable MCP photomultiplier. Owing to the very high modulation frequencies, accurate measurements could be accomplished even at short distances (6 mm) between the source fiber and detector fibers. In order to estimate the influence of the optical tissue properties, and of geometrical factors, we performed model experiments in scattering media. The time-dependence of the reemerged light appears to be characterized by three parameters: 1) a time delay, 2) a sample transit-time spread, and 3) a (complex) exponential decay. All three parameters depend on fiber distance, absorption coefficient, scattering coefficient, detector geometry, and photon wavelength. The phase-modulation data are unique in that the time delay of the reemerged photons results in phase angles as high as 300 degrees, and the transit-time spread results in a dependence of the modulation on frequency which is a faster-than-exponentially decreasing.