There are now some 80 known genetic abnormalities controlling the production of unstable hemoglobin variants, including one in the gamma chain. The clinical presentation of patients carrying these variants and the circumstances under which the variants have been detected vary considerably so that no one syndrome covers all cases, but perhaps the best general term is Unstable Hemoglobin Disease (UHD). It must be emphasized, however, that many variants which have been discovered by Hb electrophoresis, and subsequently shown to be heat unstable, do not cause hemolysis in vivo. Some examples of UHD are known as Congenital Heinz Body Hemolytic Anemia (CHBHA) because Heinz bodies are present in large numbers after splenectomy. In many clinical cases of UHD, however, Heinz bodies may not be found. This short review attempts to highlight the different clinical pictures associated with unstable hemoglobins and to discuss their diagnosis. The substitutions and deletions that are associated with heat instability of the hemoglobin molecule are listed with references to the literature.