Delusions are a product of thinking disorder. They are main importance in diagnosis of psychotic disorders. It has some discusses that how is being delusion formation, while the definition of delusion is very clear. It is necessary to understand the inner world of a patient with psychotic disorder for recognize to him/her. In order to understand the delusions, it is necessary to obtain some knowledge those meanings of the delusions, their formation what conditions, the relations between patients inner state and thinking functions, meeting what needs of patient. The knowing the delusions and their contents have influence on psychotic disorders and their outcomes. In this article, the definition, history, types, features and the formation theories of delusion have been reviewed. It has been thought that these knowledge are important from aspects such as to understand the patients, make diagnosis correctly, to apply favorable alternative treatments, to diminish disability depend on psychotic disorder. [Anadolu Psikiyatri Derg 2014; 15(2.000): 165-173]