Dyneins are high molecular weight ATPases that function as microtubule-based molecular motors. The axonemal dyneins, discovered 30 years ago, are responsible for the beating movement of cilia and sperm flagella, which they generate by producing sliding between adjacent microtubules. Cytoplasmic dynein, more recently discovered, is involved in diverse activities, including intracellular transport, nuclear migration, and the orientation of the cell spindle at mitosis.