As the mathematical formulation of the MDO problem and the process of how this formulation is implemented in the computing environment, the MDO process, also called MDO Algorithm or MDO Strategy, serves as the most central part of MDO and the most actively researched area. In terms of complex systems design, iteration is indispensable to a feasible design due to strong links between disciplines. Besides, the huge computational complexity creates an obstacle to perform an effective optimal design if this coupling between disciplines is not addressed. The MDO process enables decomposing of the large and intractable complex system design problem, converts the problem into several easy-to-handle sub-problems for optimization, and effectively coordinates the optimization of each subsystem. In addition, the decomposition shall maintain the autonomy of each discipline to make decisions on local design variables while ensuring the overall coordination of the system.