Nuclear medicine is a medical imaging specialty involving the use of radioactive compounds for diagnostic and therapeutic purposes. As a medical branch, it is considered part of Diagnostic Imaging, but differs substantially from Radiology with respect to the source of the radiation made visible by the diagnostic devices. Nuclear medicine adopts also some types of radioactive emissions for therapeutic purposes, allowing the employment of the metabolic properties of the radiopharmaceuticals for the cure of certain clinical conditions and malignant diseases. Nuclear medicine is a relatively recent discipline and owes its origins to the discovery of natural radioactivity and the development of the first instruments for medical diagnostics. From the introduction of the first gamma camera of Anger, the technology has greatly improved. The evolution has led to the development of SPECT and PET technology and in the recent years to the introduction of hybrid tomographs allowing the combination in one session of both functional and morphological images. The purpose of this textbook is to illustrate synthetically the principals of nuclear medicine diagnostics, with reference both to the technical part and main clinical indications. The booklet is addressed primarily to the degree courses for technologists, but can be reasonably used in other courses and medical training programs where there is necessity for relatively simple, yet complete and clinically relevant concepts of nuclear medicine discipline. As a complement, the manuscript will end with a dedicated section summarizing some concepts of nuclear medicine therapy.