Positron emission tomography scanning has evolved over the past 40 years from a tool used predominantly for research to a valued clinical imaging modality. Current PET scanners must perform high-quality whole-body PET and brain PET. There are several levels of PET devices from the dedicated, high-end scanners down to the hybrid PET-SPECT systems offering varying levels of performance. The next generation of PET scanners will most likely involve the use of new scintillating materials. There is also a growing interest in dedicated devices for specific applications, such as high-resolution scanners for imaging small animals.