The rigorous object-oriented analysis (ROOA) method integrates formal description techniques with standard object-oriented analysis methods. ROOA provides a systematic development process which, starting from a set of informal requirements, produces an executable objected-oriented analysis model. The model is expressed in LOTOS and provides a precise and unambiguous formal requirements specification of the static, dynamic and functional aspects of a problem. As the specification is executable, prototyping can be used to validate the specification against the requirements. Inconsistencies, omissions and ambiguities in the requirements can be detected sufficiently early in a development so that feedback can be given to the requirements capture process.