This work highlights the explosion and evolution of positron emission tomography (PET) for use in oncology research and clinical practice. 2-Deoxy-2-[F-18]fluoro-D-glucose (FDG)-PET is important in the staging of cancer, estimation of prognosis, and for its ability to predict therapeutic outcome. A number of new imaging agents are under development and may find a place in oncology when studies prove their utility. This scientific overview includes a review of the development of a number of thymidine analogs, such as 18F-3'-deoxy-3'-fluorothymidine (FLT) and 18F-1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-thymine (FMAU), including chemical structure variations; their application in a variety of tumors; and the role of various kinetic models for understanding cellular proliferation. The greatest unmet need for PET is in further developing and validating its use in the measurement of treatment response.