Drop-in alternative aviation fuels may provide a near-term opportunity to improve aviation sustainability without requiring changes in current equipment and without hindering current approaches to improving operational practices. This chapter provides background on the three elements motivating alternative aviation fuels — environment, economy, and energy security — and introduces the technology options and the requirements to fulfill demand. The motivation to reduce jet-fuel-related emissions is related not only to environmental concerns but to human health concerns. The use of market-based measures to reduce the CO2 emissions from aviation provides an additional economic motivation for alternative jet fuel use. The knowledge gained through the advancement of alternative aviation fuels would be applicable to the broader energy sector. Alternative jet fuel pathways generally produce a product slate of different fuels which can be optimized to maximize certain individual products or total output. Both commercial and military aviation have committed to alternative fuel goals.