Concluding the literature in definition, pathogenesis, nosological position and treatment of delusions we are confronted with a wide range of opinions. In the first part of the course the various definitory approaches and their value in clinical practice will be discussed. The main focus of second part of the course is dedicated to the manifold results concerning the pathogenesis of delusions, which showed that delusions are caused by complex interactions of various mental, physical and social factors. The choice of a particular delusional theme is determined by gender, age, civil status, social isolation, and special experiences ("key experiences") whereas the incorrigible conviction is based on cognitive disorders and/or emotional derailments and reinforced by social factors. But delusions cannot be longer reduced to psychopathological manifestations once established and therefore persisting. The delusional conviction is a dynamic process which only persists if disorder maintaining factors become active. These disorder maintaining factors are not necessarily corresponding with the delusion's predisposing and triggering factors. In the third part classificatory problems will be raised. Assumptions concerning nosology and classification of delusions have ranged from an independent nosological entity to the attribution to a certain mental disorder, to multicategorical classification models. Previous polydiagnostic studies indicate that delusional disorders are neither a nosological entity nor due to one particular disorder (e.g. schizophrenia) but represent nosologically non-specific syndromes which may occur superimoposed on all mental disorders. Most of the so-called primary delusions (or delusional disorders in a narrower sense - delusions not due to another mental disorder) have to be considered as diagnostic artefacts caused by the use of diagnostic criteria in particular classification systems. The final part of the course will focus on differentialdiagnostics and differentialtherapeutics. As delusions represent nosological non-specific syndromes with a multifactorial pathogenesis modern integrative treatment approaches (including psychopharmacological, psychotherapeutic and socio-therapeutic methods) have to be based on a multidimensional differential diagnosis of all the predisposing, triggering, and disorder maintaining factors. In this context the disorder maintaining factors provide the basis for effective, pathogenesis-oriented treatment of the actual symptomatology, whereas the predisposing and triggering factors provide informations for planning prophylactic long-term treatment.