Systems engineering has been defined as “the science of designing complex systems in their totality to ensure that the component subsystems making up the system are designed, fitted together, checked and operated in the most efficient way” (Jenkins, 1969). This paper documents research that reviewed three existing models for managing the complexity of the system development process in the INCOSE literature and found that while these models drew different systems of interest (SOI) from different perspectives they were unable to manage complexity in any practical manner. This paper then presents a Nine‐System Model that can be used to manage complexity. This Nine‐System Model builds in best practices and, being self‐similar, can be applied in any level of the systems hierarchy. The nine systems in the model comprise situations, processes and socio‐technical systems in a clearly defined interdependent manner. The application of the Nine‐System Model is illustrated in two examples. The paper then compares the four different models, and uses the Nine‐System Model as a framework to relate the MIL‐STD‐499 (MIL‐STD‐499A, 1974), EIA 632 (EIA 632, 1994), IEEE 1220 (IEEE 1220, 1998) and the ISO/IEC 15288 (Arnold, 2002) Standards, the SIMILAR process (Bahill and Gissing, 1998), Hitchins' version of systems engineering (Hitchins, 2007) and the problem‐solving process and shows that each is a subset of the Nine‐System Model. The paper concludes with a summary of the key benefits of the Nine‐System Model.