A low-speed diesel engine holds a unique position in large ship propulsion usage thanks to its high economic efficiency and superb reliability. However, the grounds of its reputation are not necessarily clear especially for the engineers in the other engine relating fields, since detailed and objective discussion has been rare among engines from high-speed to low-speed. At first, various indexes such as representative speed, mean piston speed and stroke-to-bore ratio are investigated among engines of various sizes. The result shows that the marine low-speed diesels run much more slowly than expected from a similarity law based on the engine size, and the ship propulsion efficiency should be a key of the ultra-low revolution. Detailed comparison between low-speed diesels and high-speed ones are also done to capture the essential differences in the engine type and structure. Finally, the application of EGR (Exhaust Gas Recirculation) to marine diesels is taken up to exemplify the difficulty in dealing the marine fuels of high sulfur content.