To ensure flight safety, it is crucial to supply engines with the appropriate quality of fuel consistently. The fuel quality is verified through proper fuel sampling methods for field and laboratory testing. This chapter outlines official standards and reviews relevant literature on fuel sampling techniques, including tools and containers designed for various sampling points. The impact of local defects—such as poorly positioned sampling valves, sloping piping sections, or tilted vessels—on sampling accuracy is also examined. Further, the chapter explains methods for obtaining representative and indicative samples from fuel lines, tanks, and filters to diagnose specific issues. Special attention is given to the unique challenges associated with sampling from aircraft fuel tanks. The chapter also addresses incidents requiring sample collection for forensic analysis, detailing solutions for areas with limited access and sampling from ground-based and aircraft filters. Lastly, the chapter provides an in-depth discussion on sampling for microbiological testing, including the associated chain-of-custody documentation required for such tests.