Hemoglobin S (HbS), which causes sickle cell disease, is now the most common genetic abnormality observed in France. Sickle cell disease is a generic term covering all the syndromes in which HbS is present. The heterozygous situation, named sickle cell trait, is perfectly well tolerated in contrast to homozygous or compound heterozygous forms, which lead to sickle cell anemia. To prevent complications, diagnosis of HbS has to be done as earlier as possible in the first day of life. This diagnosis is usually done by isoelectric focusing, or cellulose acetate electrophoresis at alkaline pH, but more and more frequently by cation exchange HPLC. It should always to be confirmed by a specific test for HbS, such as the solubility test or an electrophoresis on agar gel. In sickle cell anemia, it is important to evaluate the other factors that may modify the presentation (mostly HbF level and associated alpha-thalassemia). In a heterozygous subject presenting with pathological manifestations, more sophisticated biochemical tests or molecular biology investigations may be necessary to determine the cause of the disease.