Hematological testing is a routine aspect of health care. It is invaluable for determining the physiological status of a patient, screening for indicators of systemic disease, determining a differential diagnosis, monitoring the effectiveness of therapeutic drugs, and justifying the management plans of both ocular and systemic conditions. Since doctors of optometry continue to provide a greater spectrum of medical care to patients, it is imperative that appropriate use be made of this important clinical adjunct to the practice of primary care. This paper reviews the use of hematological testing within optometry by discussing the various elements of the complete blood count and by describing selected diseases of the eye and visual system that are associated with hematological disorders.