The complete blood count (CBC) is one of the most sensitive means of determining deviation from health in the patient as well as characterizing severity of abnormalities in the clinically obvious sick patient. This discussion on what is included in the CBC; when and why a CBC should be performed; and information about the red blood cells (erythrogram), white blood cells (leukogram), and platelets (thrombogram) to assure complete understanding, which is essential to maximize the value of the CBC.