Finding the best actuator location to control a system modelled by a partial differential equation (PDE) can improve performance and significantly reduce the cost of the control. The existence of an optimal actuator location has been established for linear PDEs with various cost functions. Various examples show that the actuator location is not only important, but should be chosen in conjunction with the controller design objective. This approach has been extended to include other aspects of actuator design, such as shape. Nonlinearities can have a significant effect on dynamics, and such systems cannot be accurately modeled by linear models. Recent research extends previous work on optimal control of nonlinear PDEs to systems where the linear part of the partial differential equation is not necessarily parabolic, and also to include actuator design. It is shown that a class of problems has an optimal control and actuator design. Under additional assumptions, optimality equations explicitly characterizing the optimal control and actuator are obtained. The results apply to optimal actuator and controller design in nonlinear structures and semi-linear wave models.