Positron emission tomography (PET) scanning with F18-fluorodeoxyglucose or FDG is a becoming a standard method for tumor staging. The prediction and evaluation of therapy response are newer applications of FDG-PET. PET often offers an early readout of treatment efficacy and is an attractive alternative to conventional anatomic assessments of treatment response. This article reviews the methods available with PET to monitor therapy response. Disease specific applications of PET imaging are then reviewed. While FDG is the most commonly used radiotracer for PET, many other radioligands could be applied in the future.