Formal thought disorder (FTD), changes in the form (as opposed to the content) of thinking, has a largely unknown pathophysiological basis or bases, although language, frontal/executive, and semantic memory mechanisms have been implicated. FTD is a major symptom of schizophrenia and is also common in mania, with there being a marked overlap of clinical features between the 2 disorders. Changes in thought form are also seen in major depression, autistic spectrum disorder, and the frontal lobe syndrome; the presentations in these latter disorders differ from those seen in schizophrenia and mania, notwithstanding some points of overlap.