Today's needs for more capable enterprise systems in a short timeframe is leading more organizations toward the integration of existing product-systems into broader intra-organizational enterprise-systems and their integration into inter-organizational systems of enterprise-systems. Although important R&D efforts lead to a general consensus that Enterprise Modeling and IT Systems Interoperability are keys to Enterprise Integration to align the human-intensive infrastructure with the information-intensive architecture, these integration issues are not handled well in industrial systems engineering practices. Bridging the gap from an integrated system to a system of interoperable systems for architecting such broader enterprise-systems or complex systems of enterprise-systems requires ‘system thinking' beyond ‘system process'. This survey paper discusses some key problems, trends and accomplishments for Enterprise Integration with a systemic approach before advocating the System of Systems (SoS) paradigm to explore some rationales and forecasts in research and engineering.