Energy economics studies energy resources and energy commodities and includes: forces motivating firms and consumers to supply, convert, transport, use energy resources, and to dispose of residuals; market structures and regulatory structures; distributional and environmental consequences; economically efficient use. It recognizes: 1) energy is neither created nor destroyed but can be converted among forms; 2) energy comes from the physical environment and ultimately returns there. Humans harness energy conversion processes to provide energy services. Energy demand is derived from preferences for energy services and depends on properties of conversion technologies and costs. Energy commodities are economic substitutes. Energy resources are depletable or renewable and storable or non-storable. Human energy use is dominantly depletable resources, particularly fossil fuels. Market forces may guide a transition back to renewable.