We apply the optimal control theory to Elliptic and Parabolic partial differential equations. We study the control problem of maximizing the net benefit in the conservation of a single species with a fixed amount of resources. We also consider an optimal control problem of a system of parabolic partial differential equations modeling the competition between an invasive and a native species. The existence of an optimal control is established and the uniqueness and characterization of the optimal control are investigated for both applications. Numerical simulations illustrate several cases. Some open problems are discussed.