This paper provides an overview of spark-ignition engine unburned hydrocarbon emissions mechanisms, and then uses this framework to relate measured engine-out hydrocarbon emission levels to the processes within the engine from which they result. Typically, spark-ignition engine-out HC levels are 1.5 to 2 percent of the gasoline fuel flow into the engine; about half this amount is unburned fuel and half is partially reacted fuel components. The different mechanisms by which hydrocarbons in the gasoline escape burning during the normal engine combustion process are described and approximately quantified. The in-cylinder oxidation of these HC during the expansion and exhaust processes, the fraction which exit the cylinder, and the fraction oxidized in the exhaust port and manifold are also estimated. Using the 1.8% of fuel for engine-out HC as a starting point, these calculations show that some 9 percent of the fuel (the precise amount will depend on engine operating conditions) escapes burning during the normal combustion process.Thus the mechanisms that result in engine HC emissions also cause a significant performance loss. While the lost opportunities for producing work, and therefore the loss in engine efficiency that these hydrocarbon emissions represent, are less by about one-quarter than the approximately 8 percent of the fuel hydrocarbons that escape the normal combustion process, they are still much more significant than has generally been supposed.These results relate to warmed-up engine operation. Additional HC emission's and performance losses occur during starting and warm-up. Experiments have been done to measure the HC emitted by the engine during starting and the early stages of warm-up with normal inlet-port fuel-injection systems. These experiments indicate that with liquid gasoline present on the walls of the intake and flowing into the cylinder in significant amounts, hydrocarbon emissions may be about 50 percent higher than HC emissions with gasoline with more sophisticated injection systems which prevent liquid film buildup. Thus during cold engine operation, the performance loss due to the fact that a significant fraction of the fuel does not burn during normal combustion is substantially higher. Significant reductions in engine-out HC-emissions will produce discernible and important improvements in engine performance and efficiency.