A unified approach is presented to the study of performance sensitivity in dynamical systems. The performance sensitivity is measured by the maximum of a performance index (p.i.). It corresponds to the worst behaviour of the system that is subject to bounded disturbances, which may be external and/or internal. This viewpoint allows one to study the sensitivity within the framework of the optimum-control theory. The concepts of static and dynamic performance sensitivity are introduced, and methods for their determination are proposed. A technique is suggested for the design of systems that are least sensitive to the effects of bounded disturbances. Examples illustrate the approach.