We study the ease of substitution between energy and other production inputs over time. We first develop a growth model with energy where our general production function allows for a nonconstant elasticity of substitution. Theoretical results show that the ease of substitution between capital and energy increases over time with the energy-capital ratio. Next, using country-level data from 108 countries between 1971 and 2011, we provide empirical evidence for a nonconstant elasticity of substitution between capital and energy. Our results imply that policies that increase the speed of the capital-energy substitution can foster long-run economic growth.