Chapter 14 develops methods for reliability-based design optimization (RBDO). Three classes of RBDO problems are considered: minimizing the cost of design subject to reliability constraints, maximizing the reliability subject to a cost constraint, and minimizing the cost of design plus the expected cost of failure subject to reliability and other constraints. The solution of these problems requires the coupling of reliability methods with optimization algorithms. Among many solution methods available in the literature, the main focus in this chapter is on a decoupling approach using FORM, which under certain conditions has proven convergence properties. The approach requires the solution of a sequence of decoupled reliability and optimization problems that are shown to gradually approach a near-optimal solution. Both structural component and system problems are considered. An alternative approach employs sampling to compute the failure probability with the number of samples increasing as the optimal solution point is approached. Also described are approaches that make use of surrogate models constructed in the augmented space of random variables and design parameters. Finally, the concept of buffered failure probability is introduced as a measure closely related to the failure probability, which provides a convenient alternative in solving the optimization subproblem.