Structural optimization can result in robotic arms with significantly improved stiffness and load carrying capacity. The geometrical shape of the manipulator links can be optimized for maximum stiffness-to-weight and strength-to-weight ratios. The problem of stiffening and strengthening a manipulator is solved by optimal redistribution of the available material without increasing the total mass of the manipulator. Since manipulators are programmed to move through a range of postures, thereby creating different loading conditions on the links, a multi-posture design criteria is implemented to provide a more uniform stiffness and strength over the range of possible postures. Finite element based performance criteria are developed which facilitate the simultaneous maximization of specific stiffness and strength. Three application examples on a SCARA class arm illustrate the dramatic potential for simultaneous improvements in specific stiffness and specific strength. The significance of multiple postures on the optimal design, the merits of tapered versus straight link shapes, and the relation of maximum stiffness to maximum strength, are also examined.