Over the past decade, a number of new kinetic modeling techniques have been developed for PET and SPECT ligands. This article will review commonly used modeling solutions for reversible positron-emission tomography (PET) and single photon emission computed tomography (SPECT) radioligands, with an emphasis on noninvasive methods. All of the modeling approaches in PET and SPECT assume a compartmental system and derive parameters that describe the compartmental system. These parameters will be defined, and their relationship to analogous parameters in pharmacology will be discussed. Then the major approaches are presented under the categories of graphical or mathematical as well as invasive or noninvasive.