This chapter provides a focused overview of acute pericarditis and constrictive pericarditis, emphasizing clinical features, diagnostic criteria, and management strategies relevant to hospitalists. Acute pericarditis is characterized by chest pain, pericardial friction rub, and ECG changes such as diffuse ST elevation, typically managed with anti-inflammatory therapy. Constrictive pericarditis involves chronic pericardial thickening and fibrosis, leading to impaired diastolic filling and signs of right heart failure. This chapter highlights key diagnostic tools, including echocardiography, cardiac MRI, and invasive hemodynamics, to distinguish constrictive pericarditis from restrictive cardiomyopathy, which presents with similar symptoms but involves myocardial stiffness rather than pericardial restriction. Differentiating features such as respiratory variation in ventricular filling pressures, septal bounce, and pericardial thickness guide diagnosis and management. Treatment options range from medical therapy for acute inflammation to pericardiectomy for chronic constriction. This chapter equips hospitalists with practical approaches to diagnosis and tailored treatment, ensuring optimal care for patients with pericardial and restrictive cardiac diseases.