Coronary artery disease (CAD) occurs when plaque builds up in the coronary arteries, narrowing them and reducing blood flow to the heart. It is the most common type of heart disease and leading cause of death in the US. Risk factors include age, family history, smoking, high cholesterol, diabetes, and physical inactivity. Over time, plaque buildup can severely block arteries and cause chest pain, heart attack, or death. Diagnosis involves ECG, cardiac catheterization, and blood tests. Treatment focuses on lifestyle changes like diet, exercise, and quitting smoking as well as medications and procedures like stents or bypass surgery to open blocked arteries.