Dystonia is mysterious and its pathophysiology is uncertain. The fundamental motor abnormality is an abnormality of muscle command signals, such that the wrong agonists may be activated for too long, there is abnormal co-contraction of agonist and antagonists, and there is excessive and misdirected action of synergists and postural fixators. The result is action dystonia. In addition, muscle spasms may occur spontaneously. The peripheral motor apparatus, the corticomotoneurone pathway, and (as far as is known) the proprioceptive feedback machinery, are all intact in primary dystonia. However, a defect of interneuronal machinery has been identified in both the brainstem and spinal cord. In blepharospasm there is hyperexcitability of the lower brainstem interneurones responsible for the R2 component of the blink reflex. In the dystonic arm there is loss of the later phases of Ia reciprocal inhibition from extensors to flexors. Both deficits may be due to loss of normal basal ganglia inputs onto interneurones. The known sites of focal lesions that may cause symptomatic dystonia all impair basal ganglia output. However, whether such abnormal interneuronal function is sufficient to explain dystonia is not known. Among many unanswered questions are 1) are the cortical instructions for movement specified correctly, and 2) what is responsible for the spontaneous dystonic spasms?