This chapter provides a detailed description of positron emission tomography. Different positron cameras by some manufacturers and their characteristics are explained and presented in tables. Detectors and PM tubes used in the PET cameras are noted. The principles of PET/CT and PET/MR scanners are described with illustrations. Factors affecting the performance of PET imaging (such as spatial resolution, sensitivity, noise equivalent count rate, attenuation correction, dead time, radial elongation, etc.) are discussed at length. Sinograms in data acquisition and time of fight technique are explained. Daily, weekly, and annual tests to perform, ensuring the accurate performance of PET cameras is elucidated.