This paper describes a number of factors which go into determining weight. The paper describes what calories are, how caloric expenditure is measured, and why caloric expenditure is different for different people. The paper then outlines the way the body tends to adjust food intake and exercise to maintain a constant body weight. It is speculated that obese people have a regulatory mechanism which makes them eat more than they need. The paper then evaluates exercise as a means of modifying body weight, but.cautions that there must be sound medical reasons to maintain a person at a colorie deficit. Diseases caused or exacerbated by obesity are listed. Norms of ideal weight are then discussed, and it is pointed out that individuals may have different biological set-points regardless of absolute weight. Finally, the paper describes procedures which accurately measure body fat. References are included. (CD)