FLIM is defined as Fluorescence Lifetime Microscopy or Fluorescence Lifetime Imaging. FLIM images consist of spatially-resolved fluorescence lifetime tau data (tau=f(x,y)). Typical FLIM images are tales-color coded where the colors reflect the tau values. FLIM images can be taken in the frequency mode (measurement of phase shift and demodulation) as well as in the time mode using streak cameras, time-gated cameras, and time-correlated single photon counting (TCSPC) units. Major FLIM applications include cell biology (e.g. FLIM-FRET to study protein-protein interactions), animal studies, artworks, microfluidics, forensic science, and clinical diagnostics. Clinical FLIM is currently applied in ophthalmology and dermatology based on TCSPC using picosecond Ti:sapphire lasers (two-photon excitation).