This chapter provides a comprehensive analysis of aircraft failures induced by fuel or fuel system issues. The failures are categorized into four main types: fuel starvation, fuel exhaustion, fuel contamination, and wrong fuel usage. The chapter explores how these failures can lead to safety incidents or accidents, supported by real-world case studies. It highlights the importance of proper fuel system maintenance, monitoring, and adherence to safety regulations. The chapter also addresses the role of international regulatory bodies, such as the International Civil Aviation Organization (ICAO), and national agencies, such as the Federal Aviation Administration (FAA) and the European Union Aviation Safety Agency (EASA), in setting certification standards for fuel systems. Despite global standards, compliance varies, and the chapter emphasizes the need for harmonized safety measures across jurisdictions. Additionally, the analysis delves into the risks posed by various contaminants—such as water, solids, and microbes—within fuel systems, and examines the dangers of using incorrect fuel types. The chapter concludes by stressing the crucial role of technical investigators and fuel engineers in conducting thorough failure analyses and implementing preventive measures to enhance aircraft safety.