Acute coronary syndrome (ACS) may not be excluded on initial evaluation and testing because of the protean presentations, up to 1/3 of patients ultimately diagnosed with ACS do not have chest pain and the limitations of existing technology. Patients with confirmed ACS or high risk should be admitted. Low-risk patients can be safely discharged from the emergency department (ED). Observation units (OUs) provide an intermediate location for evaluation and monitoring in patients with low to moderate suspicion for ACS, where serial ECGs, cardiac biomarkers, stress testing, anatomical imaging, etc. can be done.