Karl Leonhard developed the concept of cycloid psychoses by further elaborating previous findings of Carl Wernicke and Karl Kleist. A phasic alternating and recurrent as well as remittent course without residual states and a bipolar and polymorphous symptomatology are main features of cycloid psychoses. The symptom constellations are typical in a manner that, in most cases, allows establishing a diagnosis solely by carefully examining a cross-section of the clinical syndromes. 3 sub-forms are differentiated. Anxiety-happiness psychosis is characterised by extreme affective alterations with paranoid anxiety on the one hand and ecstasy with feelings of elation on the other. Formal thought disorder with incoherence, mainly as "incoherence of thematic choice", on the excited pole and perplexedness on the inhibited pole are the central elements of confusion psychosis. Motility psychosis shows hyperkinesis or akinesis of a purely psychomotor nature affecting predominantly expressive and reactive motions as main features. A description of Leonhard's highly operationalised diagnostic criteria for each sub-form of cycloid psychoses is followed by an account of the clinical significance of the concept of cycloid psychoses. Furthermore, important findings of recent research are summarised. The findings point to the fact that cycloid psychoses represent a nosological entity that can be clearly separated from schizophrenic as well as from affective psychoses. Schizophreniform, schizoaffective or acute transient psychotic disorders are also not identical with cycloid psychoses. According to the criteria of DSM-IV or ICD-10, cycloid psychoses are spread over a broad spectrum of heterogeneous diagnoses, thus hampering the recognition of the clinical and scientific importance of this homogeneous group of psychoses.