In heterogeneous environments any measurement of singlet-oxygen (1O2) luminescence becomes exceedingly difficult, the more so the analysis of its kinetics. Since the generation of 1O2 in such systems occurs at locations with different microenvironments it should be termed as an inhomogeneous process. Even more important, the diffusion of 1O2 in areas of different radiative rate constants and different solubility of oxygen has to be taken into account. Therefore, due to the extremely short 1O2 lifetimes in biological systems, the heterogeneity strongly influences the 1O2 luminescence kinetics. This chapter is dedicated to the discussion of 1O2 diffusion processes in heterogeneous systems and their consequences for the 1O2 luminescence kinetics. Starting with liposomes and continuing with cells in suspension, we introduce and discuss diffusion models, describing the 1O2 luminescence kinetics. These models demonstrate the influence of the localization of the photosensitizer on the 1O2 luminescence kinetics and intensity. The examples show that 1O2 luminescence kinetics may help to determine the exact localization of the photosensitizer. This knowledge is the pathway to a quantitative analysis of 1O2 luminescence kinetics in complex systems like cells away from the need of single-cell measurements.