Despite the huge technological advances associated with the cell blood count, in particular those derived from the hematology autoanalyzers, each generation more complete and sophisticated, the blood smear continues as the “gold standard” for the diagnosis in hematology. According to the good practices in hematology, the blood smear is indicated in all hemograms that show a deviation in the direct, indirect or calculated counts, or when there is a hematological or non-hematological entity suspected to be causing hematological manifestations, regardless of the presence of normal parameters according to the age and gender. When observing a blood smear, abnormalities associated with the erythrocytes, leukocytes and platelets can be detected. The present module will analyze the most important aspects of the red cell morphology, including cell-cell interactions, such as rouleaux formation and agglutination, as well as the concepts of anisocytosis, poikilocytosis, anisochromasia and dimorphism; variations in size, in particular macrocytosis and microcytosis; variations in the shape including target cells, stomatocytes, dacryocytes, drepanocytes, elliptocytes, spherocytes, mushroom-shaped cells, spiked cells, keratocytes and schisocytes; red cell inclusions such as circulating erythroblasts, basophilic stippling, Howell-Jolly bodies; and variations in the hemoglobin contents, in particular hypochromic red cells and polychromatophilia. Specific elements will be analyzed in detail and a clinical correlation with associated entities will be discussed. This first module is dedicated to the analysis of the erythrocyte morphology in the blood smear and provides the laboratory technologist with the tools necessary to adequately identify the different findings, as well as the physician with the information to associate such findings with the clinical entities.