The Royal Park Multidiagnostic Instrument for Psychosis (RPMIP) is a validity-oriented assessment procedure developed for the acute psychotic episode using serial interviews and multiple information sources to construct a data base of clinical information. A number of sets of operational criteria, including 11 definitions of schizophrenia and several concepts of atypical, schizoaffective, and affective psychoses, are simultaneously applied to the data base to produce a diagnostic profile for each patient that can be linked to other variables. This article describes the rationale for the development of the RPMIP and contrasts it with other assessment and diagnostic procedures. A companion article (Part II) presents data on interrater reliability and procedural validity, together with an account of the structure and development of the instrument.