Optimal design of large systems is easier if the optimization model can be decomposed and solved as a set of smaller, coordinated subproblems. Casting a given design problem into a particular optimization model by selecting objectives and constraints is generally a subjective task. This article describes how such a subjective selection can be made so that the resulting optimal design model can be directly partitioned into an appropriate decomposed form. This process is termed decomposition synthesis. A particular methodology for synthesizing hierarchically decomposed optimal design models is presented together with examples.