Abstract Dynactin is a 1·2 mDa protein complex initially isolated as an activator of cytoplasmic dynein-driven vesicle movementin vitro. Genetic studies have verified that dynactin participates in dynein-based transport events as varied as nuclear localization and axonal transport. Recent work has focused on identification of the 10 different polypeptide subunits of dynactin and elucidation of the way in which they are organized within the dynactin molecule. A picture is beginning to emerge as to how this complicated structure interacts with dynein and cytoplasmic organelles to form an assembly capable of motility.