Fluorescence lifetime imaging microscopy (FLIM) technology has so many outstanding and unique advantages compared with other fluorescence imaging methods that it attracts more and more researchers in biomedicine engineering field. Frequency domain modulation method and time domain methods including time-gated detection and time correlated single photon counting (TCSPC), are the main ways to realize FLIM. Principles, current reaseach situation and achievements of these methods are presented as well as the comparison of their main performance parameters. Wide-field FLIM is more suitable for delay imaging. Fluorescence lifetime polarization resolved imaging and endoscope FLIM are both promising research areas of FLIM.