In this work some progressive object–oriented requirement definition methods were analyzed (ICONIX, Reggio, Scores). Rules and principles of these methods were generalized and comprehensive content of requirements model was presented. Requirements definition must define state and behavior of target information system independently of future design. Requirements model was proposed where use cases are presented as interfaces together with associated classes of problem domain, operations are specified with signatures, pre and post conditions wich must be written in formal language OCL. Model was described using UML class, sequence diagrams, state charts and OCL constraints and is illustrated with examples from case study.