Fuel is required for an engine to produce work. Reciprocating internal combustion engines are classified by the manner by which this fuel is ignited: spark ignition (SI) or compression ignition (CI). The specific effects of changing a given fuel property depend on many detailed parameters of the engine and combustion strategy in which the fuel is used, as well as the conditions over which the engine is operated. SI engines typically perform better with higher volatility fuels that resist autoignition (gasolines), while conventional CI engines tend to perform better with fuels exhibiting the opposite volatility and autoignition characteristics (diesel fuels). Fuels and engine technologies are currently evolving, however, and as a result, these historical fuel distinctions may become less relevant in the future. As such, it is important to understand how the chemical composition of a fuel determines its operational attributes, including autoignition quality and volatility, as well as many others. This chapter provides a broad overview of topics related to the selection and use of fuels for reciprocating internal combustion engines, including the following: petroleum‐based, alternative, and renewable fuel production techniques; the compositional characteristics of liquid‐ and gas‐phase fuels; key physical and chemical properties of fuels; and the specifications that have been developed to ensure that fuels perform as required in their given applications. The chapter concludes with a brief summary and outlook for the future of fuels for reciprocating internal combustion engines.