Dystonia is a movement disorder with many presentations and diverse causes. A systematic approach to dystonia helps to ensure that patients with this disorder receive optimum care. This Review begins with a summary of the clinical features of dystonia, followed by a discussion of other disorders to be considered and excluded before assigning the diagnosis of dystonia. Next, we emphasise the importance of classifying dystonia along several dimensions, and we explain how doing so aids in narrowing the differential diagnosis. The more common forms of dystonia are discussed in detail. Finally, we describe how to apply the clinical information for selection of appropriate laboratory investigations.