The development of the medium speed four stroke diesel engines in recent years has made a considerable progress. Utilization of poorer grades of heavy fuels was made possible, even for the smaller engines of this range. The mean effective pressures were increased for newly developed engines and also for many of the existing types.The most significant trend is the continued search to reduce the specific fuel consumption. This can be achieved by several modifications of the conventional engine designs, improving the combustion process which will in most cases lead to higher firing pressures.Consequently the thermal and mechanical loading of the pistons is higher than previously. The resulting stresses demanded new types of pistons, the required properties of which are described. A review is given of the design and material of these pistons and their individual characteristics.Advanced research methods like three dimensional finite element analysis, careful stress measurements and sophisticated designs must be assisted by a high level of quality in the production.Experience from pistons in heavy fuel operation is reported concerning wear rates and general behaviour. The outlook on future piston development is based on the expected engine parameters, the changes in future fuel and lube oil and on the demands placed by the engine users.