The object-process analysis (OPA) methodology combines ideas from object-oriented analysis (OOA) and data flow diagrams (DFD) to model both the structural and procedural aspects of a system in one coherent frame of reference. This is contrary to conventional object-oriented approaches, that use different tools to describe the structure and the behaviour of the system. The underlying observation of the OPA paradigm is that every thing in the universe of interest is either an object or a process, and that a process is not necessarily a method of a single object class. This opens the door for the possibility of modelling systems so that both their structural and procedural relations are represented within the same frame of reference, without suppressing each other. The two major differences between OPA and OOA are OPA's detachment of processes from objects and its recursive scaling capability. The OPA methodology has proven to model faithfully complex systems, such as computer integrated manufacturing, documentation and inspection and an intelligent computer-assisted instruction shell. This work lays down the foundations of OPA. It provides concise definitions of the basic building blocks of the method: objects, processes, classes, features, and the structural and procedural relations among them. The object-process diagram (OPD) is presented as an effective visualization tool that incorporates elements from both DFD and OOA. Due to synergy, both the information content and expressive power of OPD are greater than those of DFD and OOA diagrams combined.