The aim of the 4th edition of the Atlas of Nuclear Cardiology is to provide physicians and students in cardiology, radiology, and nuclear medicine who want the latest information in the field of cardiovascular nuclear medicine up-to-date and comprehensive information on advances in instrumentation, radiotracers, protocols, and clinical studies. Unlike other books that are narrow in their scope of either technology and technique or clinical studies, the 4th edition of the Atlas will present diagnostic algorithms and schematic diagrams integrated with nuclear cardiology procedures generously interspersed with color illustrations to emphasize key concepts in cardiovascular physiology, pathology, and metabolism relevant for the clinical practice of cardiology. The atlas emphasizes todays most current information, meeting the requirements for those who will be using the book as a reference source for certifying or re-certifying in cardiology, nuclear cardiology, nuclear medicine or radiology. Hybrid PET/CT and SPECT/CT represent new technologies that were introduced recently in clinical medicine and are evolving rapidly with several improvements in instrumentation, imaging procedures as well as in clinical trials that support the expanded role of these technologies in clinical practice. As such, an updated 4th edition of the Atlas is critical in order for the clinicians remain current with the imaging field and maintain their skills. Imaging protocols with the se technologies have to be updated and/or expanded in order to acquire high quality images at a reduced radiation burden to the patient while advancing the application of these techniques for more advanced disease detection. Accordingly, beyond significantly updating the chapters from the 3rd edition, 2 new chapters will be introduced in the 4th edition, which reflects the expanded clinical applications of the technologies in the past 3 years. The new chapters are as follows: "Hybrid SPECT/CT and PET/CT Imaging" and a dedicated chapter on "Radiation Safety and Exposure: Clinical Decision-Making and the Risk-Benefit Ratio". Chapter 7 from the 3rd edition will be deleted. The updated Atlas will serve as a reference source for all cardiologists, radiologists, and nuclear medicine physicians interested in the most up-to-date approaches to noninvasive diagnostic cardiovascular nuclear imaging techniques for the evaluation of patients with known or suspected coronary artery disease as well as non-coronary heart disease. It will also serve as a ready reference textbook for medical students and residents interested in the practice of cardiovascular medicine.