Neutropenia is defined as an absolute neutrophil count of less than 1,500 per μL in adults and children older than 1 year, and less than 1,000 per μL in infants. Neutropenia can be acquired or inherited. It is classified as mild (1,000–1,500 per μL), moderate (500–999 per μL), or severe (less than 500 per μL). Patient presentation can range from asymptomatic to severe illness requiring hospitalization. Acquired neutropenias and their causes include autoimmune neutropenia, chronic idiopathic neutropenia, chemotherapy-induced neutropenia, febrile neutropenia, hematologic malignancy, idiosyncratic drug-induced neutropenia, infection-related neutropenia, and nutritional deficiency (ie, vitamin B₁₂, folate, and copper). Inherited neutropenias and their causes include bone marrow failure, cyclic neutropenia, and severe congenital neutropenia. Genetic testing may be required for diagnosis. Acquired neutropenias are treated by addressing the underlying etiologies. Some patients experiencing recurrent severe infections may benefit from granulocyte colony-stimulating factor. Most inherited neutropenias are treated with granulocyte colony-stimulating factor. Febrile neutropenia is an oncologic emergency. It is defined as a single oral temperature of 101°F or greater or a temperature of 100.4°F or greater sustained for 1 hour with an absolute neutrophil count of less than 500 per μL. Febrile neutropenia warrants emergent evaluation, but low-risk patients with a malignancy who meet strict criteria can be treated as outpatients.