Hemoglobin polymerization develops from a process of condensation, molecular interaction, aggregation, and assembly. Molecular organization of structures formed during polymerization depends on the type of hemoglobin, its specific defect, and its state of oxygenation. Sickling exemplifies hemoglobin polymerization. In the reduced state, sickle hemoglobin (HbS), alone or in combination with other hemoglobins, undergoes condensation, aggregation, and assembly into long rod-like structures. Oxygenated or reduced normal and abnormal hemoglobins and oxy-HbS also polymerize; they form aggregates, microtubules, or crystals rather than rods. Crystals form from aggregates, microtubules or rods, but molecular reorganization of the earlier structures into a new lattice is required. Thus, sickling is due to sol-gel transformation rather than crystallization, and polymers of HbS are unique because they differ in molecular organization from hemoglobin aggregates, microtubules, and crystals.