Apparatus for analysing a biological substance comprises a probe (1) for directing light emitted by a light source onto an area of the substance to cause fluorescence at that area. The fluorescent light is received by receiving means in the probe and is conveyed to spectral analysis means (4; 1-8; 2-8) for use in analysing the spectrum of the received fluorescent light and temporal analysis means (8, 10; 118) for analysing temporal characteristics of the received fluorescent light. The use of common receiving means ensures that the temporal and spectral analyses are conducted on the same area, and both types of analysis can, in combination, provide more information on the area being analysed than each individual type of analysis. Apparatus for analysing a biological substance by causing a substance to fluoresce and measuring characteristics of the resultant fluorescent light may have spectral analysis means and temporal analysis means, wherein the spectral analysis means is connected to the temporal analysis means so as to select the wavelength of the received light to be analysed by the temporal analysis means.