There are many causes of thrombocytopenia in infants and children. With experience, they can be readily identified and appropriate treatment instituted, if required. Particular attention should be paid to the well baby with unexplained thrombocytopenia, as alloimmune thrombocytopenia is the most common cause of intracranial hemorrhage due to thrombocytopenia and can be prevented if identified and treated early on. Autoimmune thrombocytopenia of childhood is generally benign, but children may require careful management until spontaneous resolution occurs. Human immunodeficiency virus infection should be looked for in these patients.