Positron emission tomography (PET) plays a fundamental role in medical imaging, with a wide range of applications covering, among the others, oncology, neurology and cardiology. PET has undergone a steady technological evolution since its introduction in mid 20th century, from the development of 3D PET in the late 1980s, to the invention of PET/CT in the 1990s and more recently with the introduction of PET/MR scanners. After illustrating briefly the historical evolution of PET imaging, this presentation will describe the state of the art of PET scanners in use nowadays, analyzing their performance and limitations, and will illustrate the main topics of research currently active in the field.