Since the wide adoption of Internet technologies in manufacturing automation, integration has become one of the most advertised trends in the field. The concept of integration in automation is by no means new. It was first formulated in the mid 1970s with the computer-integrated manufacturing (CIM) idea. Its aim was to integrate several already existing islands of computer-assisted manufacturing subsystems to a comprehensive and transparent, enterprise-wide information processing system. This article sketches the roots as well as the historical evolution of the integration idea and describes the pieces it takes to complete the picture of integration with its horizontal, vertical, and temporal aspects. Emphasis is put on building blocks like networks and their interconnections, middleware technologies, and application software frameworks. Standardization issues are addressed as well as future challenges in the field