As organizational systems evolve into complex, distributed, multi-platform environments, the need for interoperability has become a central concern in enterprise integration. Middleware—software that enables communication, data exchange, and coordination between heterogeneous systems—plays a foundational role in achieving interoperability. This theoretical review synthesizes key middleware theories, including distributed systems theory, service-oriented architecture, enterprise integration theory, and interoperability frameworks. A Middleware Interoperability Theoretical Framework (MITF) is proposed, illustrating how middleware facilitates syntactic, semantic, and process-level interoperability. Conceptual diagrams, architecture layers, and detailed analysis of middleware functions are included. The paper concludes with implications for organizations, systems architects, and future research directions.