A general model for the valuation of natural resource investments is formulated and analyzed within a stochastic control theoretic framework. Using dynamic programming, the value of such an investment with a general payoff function is determined under the assumption that the commodity price process is given by a stochastic differential equation. The analysis results in closed form analytic solutions which can easily be computed and exhibits qualitatively different optimal behaviors, depending on parameter values. Implications for stocks and options are also considered.