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This paper reviews the literature on deposits that form on all internal surfaces coming into contact with the fuel in a gasoline engine. It does not consider deposits that can form in areas primarily affected by the engine lubricant. The physical, and especially the chemical, mechanisms that lead to deposit formation are very complex and poorly understood, and are different in different parts of the engine. Deposits can impair engine operation in terms of fuel economy, power emissions, octane requirement and driveability. In the worst cases they could lead to engine damage. Deposits in most parts of the engine can be controlled through the use of fuel additives. Test procedures used in deposit-related studies are characterised by poor repeatability and reproducibility because of the complex nature of the processes involved. Deposit formation and, consequently, the effectiveness of deposit control additives also depend on base fuel properties and engine design and operation.