The determination of fluorescence lifetime requires only relative measurements of intensity and so is especially useful for biomedical samples in which the heterogeneous nature of tissue and autofluorescence cause significant problems. Since fluorescence lifetime is dependent upon both radiative and non-radiative decay rates, it may be used to distinguish between different fluorophore molecules (with different radiative decay rates) and to monitor local environmental perturbations that affect the non-radiative decay rate. Fluorescence lifetime probes have been demonstrated for many biologically significant analytes including [O2], [Ca2+] and pH. Fluorescence lifetime imaging (FLIM) can be applied to almost any optical imaging modality, including microscopy and potentially to non-invasive optical biopsy. Fluorescence lifetime data may be acquired in the frequency or time domain. The recent development of user-friendly and relatively portable ultrafast laser technology and the availability of ultrafast gated optical image intensifiers (GOI’s) enable the development of potentially inexpensive time domain FLIM instruments that may be deployed outside specialist laser laboratories.