Compared with the widely used intensity-based method, the greatest advantage of the lifetime-based method is that a relation between the luminescent lifetime and pressure is not dependent on the illumination intensity. Therefore, the problem associated with non-uniform illumination in the intensity-based method becomes essentially irrelevant to the lifetime method. Theoretically speaking, lifetime measurement is also insensitive to luminophore concentration, paint thickness, photodegradation and paint contamination; thus a wind-off reference intensity image (or signal) is not required and the troubles associated with model deformation do not exist. The lifetime method for PSP and TSP can be applied to both a laser scanning system and an imaging system. Davies et al. (1995) developed a pulsed laser scanning system to directly determine the luminescent lifetime and used it to measure the pressure distributions on a cylinder in subsonic flows and on a wedge at Mach 2. Torgerson et al. (1996) developed a portable, modulated, two-dimensional laser scanning system that can simultaneously measure both the luminescent intensity and phase angle; this system was used to measure the surface pressure distributions in a low-speed impinging jet and on an airfoil in transonic flow. The system was further refined by Lachendro et al. (1998) and used to measure the pressure distributions on a wing of a Beechjet in flight tests. A fluorescent lifetime imaging (FLIM) system for PSP and TSP has become promising as solid-state imaging technology makes a rapid advance. The FLIM system, originally proposed by biochemists for oxygen detection in a small area (Szmacinski and Lakowicz 1995; Hartmann and Ziegler 1996), was used for PSP measurements in wind tunnels at DERA (Holmes 1998). DERA’s FLIM system comprised a phase-sensitive camera, modulated blue LED array, associated control hardware and computer. This Chapter discusses the response of the luminescent emission to a time-varying excitation light and describes the luminescent lifetime measurement techniques, including the pulse method, phase method, amplitude demodulation method and gated intensity ratio method. Although the discussion is focused on PSP, these techniques are generally applicable to TSP as well. Measurement uncertainty of the lifetime methods is discussed in Chapter 7. Similar analyses of the lifetime-based techniques were given by Goss et al. (2000) and Bell (2001).