Microcytic anemia is defined as anemia with a mean corpuscular volume (MCV) of less than 80 mcm3 in adults. Age-specific parameters should be used for patients younger than 17 years. The cause of microcytic anemia includes acquired and congenital causes, which should be considered separately according to the age of the patient, risk factors, and coexisting signs and symptoms. The most common cause of microcytic anemia is iron deficiency anemia; it can be managed with oral or intravenous iron, depending on the severity and comorbid conditions of the affected individual. Pregnant patients and patients with heart failure with iron deficiency anemia require special considerations to prevent significant morbidity and mortality. The wide spectrum of thalassemia blood disorders should be considered in patients with a particularly low MCV in the absence of systemic iron deficiency. Iron chelation may be required for some of these patients. Sickle cell anemia and sideroblastic anemia are important inherited causes of microcytic (as well as normocytic) anemia. Promising treatments are being developed for patients with transfusion-dependent thalassemia and sickle cell anemia.