This chapter provides a practical, evidence-based framework for hospitalists to diagnose and manage acute coronary syndrome (ACS), emphasizing rapid risk stratification, appropriate use of troponin testing, ECG interpretation, and guideline-directed medical therapy. It outlines the diagnostic criteria for ACS subtypes—unstable angina, non-ST-segment elevation mycocardial infarction (NSTEMI), and STEMI—highlighting the importance of integrating clinical presentation with dynamic ECG changes and serial troponin trends. A key focus is distinguishing between acute myocardial injury and type 2 myocardial infarction (MI): while both conditions may present with troponin elevation, type 2 MI is defined by a mismatch in oxygen supply and demand (e.g., sepsis, anemia, tachyarrhythmia) with clinical evidence of ischemia, whereas nonischemic acute myocardial injury (e.g., myocarditis and renal failure) lacks ischemic symptoms or ECG changes. The chapter stresses the importance of context and comorbidities in diagnosis, guiding hospitalists on when to involve cardiology, pursue invasive evaluation, or opt for conservative management. Ultimately, the goal is to improve outcomes through accurate diagnosis, risk-appropriate treatment, and timely disposition planning.