The paper comprises brief descriptions of three distinctive types of marine Diesel engine, and some expressions of opinion upon a number of matters of current interest. The types considered are double-acting, two-stroke engines; single-acting, two-stroke, crosshead engines; and single-acting, two-stroke, trunk engines. The author restricts his remarks to some of the engine types with which, for many years past, his work has lain. Some matters of detail dealt with are the timing of cylinder lubricators, piston rings, pistons and liners, piston rod stuffing boxes, piston rod scraper boxes, feed pumps and valves, scavenge blowers and lubricating oil systems. Indicated horse-power and mechanical efficiency also find a place, together with crankcase safety precautions, vibration and noise, and materials of construction. The author also deals with surface finish, the effect of design changes upon maintenance, the use of lower-grade fuel oils, scavenging, and engine rating, and concludes with a reference to the comparison of Diesel and steam machinery and tables of typical sea-service results of a 6,000 s.h.p., double-acting, two-stroke engine.