The chapter characterizes the optimal water policy, i.e., the feasible policy (under physical, institutional, and budgetary constraints) that generates the highest payoff (social welfare) to the water economy. The optimal policy involves water allocations from the different sources to the different sectors/users and the investments in infrastructure needed to carry out these allocations. It is shown that the optimal policy begins with an infrastructure construction stage (which is usually short), followed by a turnpike stage that eventually converges to a steady state, during which water allocations, capital stocks, and investment rates remain constant over time.