The advantages of using extended expansion-strokes are reviewed together with current methods for achieving this. A supporting thermodynamic analysis is presented for both diesel and spark-ignition (Otto) engine cycles employing extended expansion-strokes. It is also shown that such cycles do not generally preclude the use of turbo-charging since sufficient residual pressure prevails in the cylinders at the point of exhaust valve opening for turbo-charger operation. A basic, relatively simple, mechanism, and variants thereof, is described that permits the use of extended expansion strokes. It was concluded that the use of an extended expansion-stroke need not result, for a prescribed swept volume, in a significant increase in engine envelope volume or friction losses nor, in some cases, engine complexity. It was also concluded that extended-expansion-stroke engines in which the ratio of the compression and expansion strokes is a variable can be arranged to have, by virtue of setting the compression and expansion strokes equal, a maximum power output the same as that of an otherwise comparable conventional engine whilst benefiting from an improved thermal efficiency, at part load, when the expansion stroke is arranged to exceed that for the compression process.