Positron Emission Tomography is a nuclear medicine technique first used to study the brain. Several decades ago, PET scanners design and performance have improved considerably: number of detectors has increased from 20 to 20,0000, axial field of view from 2 to 20 cm, spatial resolution has improved from 25 to 5 mm, sensitivity has increased of about 1000 fold. At the same time, clinical applications have grown dramatically. In the first section of this book the authors review some of developments in PET instrumentation, with emphasis on data acquisition, processing and image formation. In the second section authors expose examples of applications in human research. In the last section authors describe applications in assessment and prediction of oncological treatment response.