This chapter introduces the physical processes involved in the generation of fluorescence and how this links to the fluorescence lifetime of a molecule. It considers how the lifetime is affected by the local environment before describing the methods used to generate fluorescence lifetime images. In the longer term, arrays of single photon avalanche photodiodes are likely to become the most common detector used for Fluorescence Lifetime Imaging Microscopy. For every molecule there is an inherent fluorescence lifetime determined by the electronic structure of the molecule and the associated quantum mechanical rules. The other major effect on the fluorescence lifetime of certain molecules is a combination of the physical and chemical environments. The pulse excites the fluorescence in the sample and this fluorescent light is collected and detected by a high-speed detector. The returned fluorescence light then comes back through the dichromatic filter and onto the camera.