Function, structure, and behavior are the three major facets of any system. Structure and behavior are two inseparable system aspects, as no system can be faithfully modeled without considering both in tandem. Object-Process Methodology (OPM) is a systems paradigm and language that combines structure–behavior codesign requirements with cognitive considerations. Based on the formal mathematical foundations of graph grammars and a subset of natural language, OPM caters to human intuition in a bimodal way via graphics and autogenerated natural language text. In a nutshell, OPM processes transform objects by creating them, consuming them, or changing their states. The concurrent representation of structure and behavior in the same, single diagram type is balanced, creating synergy whereby each aspect helps understand the other. This chapter defines and demonstrates the principles and elements of OPM, showing its benefits in facilitating structure–behavior codesign and achieving formal, semantically sound, and humanly accessible conceptual models of complex systems in a host of domains and at virtually any complexity level.