There are numerous ways of classifying the causes of anemia, and no one way is necessarily superior to another. It is equally important to appreciate the differences in the approaches to diagnosis between children and adults, men and women, and persons of different ethnic backgrounds. Regardless of the specific algorithm followed in evaluating anemia, it is essential that easily remediable causes such as nutritional deficiencies, hemolysis, and anemia of renal insufficiency are identified early and treated appropriately. In general, the differential diagnosis of anemia can be substantially narrowed by subcategorization into "microcytic," "normocytic," and "macrocytic" subtypes on the basis of mean corpuscular volume. However, such classification is a starting point and not infallible. Each category then can be deciphered using a stepwise approach that utilizes readily accessible laboratory tests.