We report the investigation of Foerster's Resonance Energy Transfer dynamics in GFP based tandem constructs in living T-cells using a combination of Fluorescence Lifetime Imaging Microscopy (FLIM) and Fluorescence Lifetime Micro-Spectroscopy (FLMS) at picosecond time resolution and nanometer spectral resolution. The involvement of multiple lifetimes of CFP in energy transfer was analyzed by plotting pre-exponential factors of individual lifetimes along the wavelength resulting in the Decay Associated Spectra (DAS). A change in the amplitude of pre-exponential factors from positive to negative at the acceptor emission maxima was used as a confirmation of FRET in the multiexponential lifetime analysis.