The purpose of this research is to develop a general, rigorous approach for robust optimal design. This methodology will allow a designer to explicitly consider and control the effects of variability in design variables and parameters on a design. Variability is defined in terms of tolerances which bracket the variation of fluctuating quantities. Both linear and nonlinear tolerances analysis will be studied. The methodology for feasibility robustness will be validated by applying it to ten engineering design problems. The problems will be chosen to provide examples with applicability to a broad spectrum of engineering fields.//