Cardiology (from Ancient Greek καρδίᾱ (kardiā) 'heart' and -λογία (-logia) 'study') is the study of the heart. Cardiology is a branch of medicine that deals with disorders of the heart and the cardiovascular system, and it is a sub-specialty of internal medicine. The field includes medical diagnosis and treatment of congenital heart defects, coronary artery disease, heart failure, valvular heart disease, and electrophysiology. Physicians who specialize in this field of medicine are called cardiologists. Pediatric cardiologists are pediatricians who specialize in cardiology. Physicians who specialize in cardiac surgery are called cardiothoracic surgeons or cardiac surgeons, a specialty of general surgery.

All cardiologists in the branch of medicine study the disorders of the heart, but the study of adult and child heart disorders each require different training pathways. Therefore, an adult cardiologist (often simply called "cardiologist") is inadequately trained to take care of children, and pediatric cardiologists are not trained to treat adult heart disease. Surgical aspects outside of cardiac rhythm device implant are not included in cardiology and are in the domain of cardiothoracic surgery. For example, coronary artery bypass surgery (CABG) and surgical valve replacement are surgical procedures performed by surgeons, not cardiologists. Typically, a cardiologist would first identify who is in need of cardiac surgery and refer them to a cardiac surgeon for the procedure. However, an increasing number of structural interventions can be performed less invasively by cardiologists; these include coronary procedures such as cardiac catheterization and percutaneous coronary intervention; structural valve procedures such as transcatheter aortic valve implantation (TAVR) and MitraClip; and procedures for heart rhythm disorders such as pacemaker implantation, Implantable cardioverter-defibrillator (ICD) insertion, and ablation.

Cardiology is a specialty of internal medicine.

To become a cardiologist in the United States, a three-year residency in internal medicine is followed by a three-year fellowship in cardiology. It is possible to specialize further in a sub-specialty. Recognized sub-specialties in the U.S. by the Accreditation Council for Graduate Medical Education are clinical cardiac electrophysiology, interventional cardiology, adult congenital heart disease, and advanced heart failure and transplant cardiology. Cardiologists may further become certified in echocardiography by the National Board of Echocardiography, in nuclear cardiology by the Certification Board of Nuclear Cardiology, in cardiovascular computed tomography by the Certification Board of Cardiovascular Computed Tomography in cardiovascular MRI by the Certification Board of Cardiovascular Magnetic Resonance. Recognized subspecialties in the U.S. by the American Osteopathic Association Bureau of Osteopathic Specialists include clinical cardiac electrophysiology and interventional cardiology.

In the United Kingdom, following completion of two years of foundation training and three years of internal medicine training, doctors may commence the five-year cardiology specialty training programme. As cardiologists are physicians, all prospective cardiologists must complete the Membership of the Royal Colleges of Physicians of the United Kingdom (MRCP (UK) diploma, prior to commencing cardiology specialty training. It is possible to specialise in a recognised sub-specialty; the core sub-specialties are interventional cardiology, electrophysiology, cardiac imaging, heart failure, and adult congenital heart disease. All cardiologists must complete a core curriculum in cardiology, and may undertake further training and additional qualifications and certifications necessary for their chosen sub-specialty, before being awarded the Certificate of Completion of Training (CCT) in cardiology.

In India, a three-year residency in General Medicine or Pediatrics after M.B.B.S. and then three years of residency in cardiology are needed to be a D.M. (holder of a Doctorate of Medicine [D.M.])/Diplomate of National Board (DNB) in Cardiology.^{[citation needed]}

Per Doximity, adult cardiologists earn an average of $436,849 per year in the U.S.

Cardiac electrophysiology is the science of elucidating, diagnosing, and treating the electrical activities of the heart. The term is usually used to describe studies of such phenomena by invasive (intracardiac) catheter recording of spontaneous activity as well as of cardiac responses to programmed electrical stimulation (PES). These studies are performed to assess complex arrhythmias, elucidate symptoms, evaluate abnormal electrocardiograms, assess risk of developing arrhythmias in the future, and design treatment. These procedures increasingly include therapeutic methods (typically radiofrequency ablation, or cryoablation) in addition to diagnostic and prognostic procedures.