The plethora of presently available prosthetic knee components can be divided into two groups based on how they are controlled: recent innovations that incorporate an onboard computer and the more familiar purely mechanical devices. These categories then can be subdivided into generic functional classes based on the degree of stance phase stability and swing phase responsiveness offered by each type of knee mechanism. This article summarizes the key advantages and limitations of available prosthetic knee systems and suggests a simple method to match the biomechanical capabilities of specific prosthetic knee components to the individual functional capabilities and goals of the person with an amputated limb.