Cardiologists have extensive knowledge of the cardiovascular system, including the heart, blood vessels, and circulation. Cardiologists use a variety of diagnostic tools and tests to evaluate the health of the heart, including electrocardiograms (ECGs), stress tests, echocardiograms, and angiograms. Common cardiology conditions include coronary artery disease, heart failure, arrhythmias, valvular heart disease, and congenital heart defects.