Engine friction serves as an important domain for study and research in the field of internal combustion engines. Research shows that friction between the piston and cylinder accounts for almost 20% of the losses in an engine and therefore any effort to minimize friction losses will have an immediate impact on engine efficiency and thus vehicle fuel economy. The two most common methods to experimentally measure engine friction are the floating liner method and the instantaneous indicated mean effective pressure (IMEP) method. This paper provides a detailed review of the IMEP method, presents major findings, and discusses sources of error. Although the instantaneous IMEP method is relatively new compared to the floating liner method, it has been used by many scientists and engineers for calculating piston ring assembly friction with consistent results. It should be noted that irrespective of the method utilized, most of the experiments are conducted under motoring conditions and limited results are available for fired operation. The instantaneous IMEP method is based on the concept of force balance. The forces on the piston such as combustion gas pressure, crank case pressure, side thrust, as well as piston and connecting rod inertial forces are used to determine the friction force between the piston and the cylinder. There is no major engine modification required for the instantaneous IMEP method and thus is less expensive than floating liner and can be adapted to production engines. However, the friction force is relatively small compared to other forces acting on the piston such as cylinder gas pressure and inertial forces. Therefore, this method can be prone to large errors if the force measurements are inaccurate. Previous studies have revealed that friction in an engine is not a simple phenomenon to measure due to the presence of all three types of lubrication regimes including boundary, mixed, and hydrodynamic. The importance of lubricating oil and various additives further increases as it affect oil film thickness, piston ring tension, temperature, and viscosity, which are responsible for determining the nature and amount of friction in an engine. Thus, selection of lubricating oil and the proper additive is a decisive factor in controlling the amount of friction loss in an engine.