If a person were to define what a reciprocating internal combustion engine does the simplest definition would be a device that turns chemical energy into heat, heat into pressure and pressure into reciprocating mechanical work. Horsepower would be the unit of measure of that mechanical work. Just about every car ad in every magazine mentions horsepower, people talking about their cars bandy the word about and even most lawn mowers have a big sticker on them to tell you the horsepower rating. But what is horsepower, and what does the horsepower rating mean in terms of performance? This paper will discuss three important things pertaining to horsepower: -What is horsepower -Why it is important -How to properly measure horsepower If a company buys a 2000 hp engine, they should expect it to make 2000 hp. If you buy a 2000 hp engine and attempt use 2200 hp you can probably expect it to break prematurely. Thus the term, make it or break it.