With today's multi-million gate FPGAs, the complexity of the designs within these devices is becoming a significant challenge. When creating scalable systems based around multiple FPGAs, this creates further complexity and challenges to the systems engineer for creating final product. This presentation looks at the options of building scalable systems and the mechanisms available for communication within these systems. It discusses: why complex systems need FPGAs; requirements for complex FPGA systems; and the design, operation and management of complex FPGA-based systems.