A reliability-based Level I design procedure is proposed for ring-stiffened cylindrical shells under external pressure. The main collapse modes and the present safety factor approach are reviewed. The major aspects involved in code development, such as the statistical properties of the basic variables, reliability methods, sensitivity studies, code format, target reliability, partial safety factor optimization, and comparison with the present practice are described. Safety margins for design are proposed which depend on the design pressure, maximum expected overdive, and shell slenderness. Some suggestions for future work are also given.