In this chapter, we explain how positron emission tomography (PET) data are analyzed to estimate behavior of radiotracer injected. For this purpose, compartmental model and its kinetic parameters are introduced, and concept, mathematical basis, and biological interpretation of the compartmental model and the kinetic parameters are described. General form of the compartmental model is brought for comprehensive understanding of the model. Several approaches for fast estimation of the kinetic parameters are introduced based on general compartmental model. The input function for the compartmental model is important for quantitative analysis of PET data, and the reference region model is one approach to avoid acquisition of the input function.