Bodybuilding is the sport of developing muscle fibers through the combination of weight training, increased caloric intake, and rest. Competitive bodybuilders display their physiques to a panel of judges, who assign points.

The sport is not to be confused with strongman competition or powerlifting, where emphasis is on actual physical strength, or with Olympic weightlifting, where emphasis is equally split between strength and technique.

Though superficially similar to the casual observer, the fields entail a different regimen of training, diet, and basic motivation. Bodybuilders aspire to develop and maintain an aesthetically pleasing (by bodybuilding standards) body and balanced physique. A bodybuilder's size and shape are far more important than how much he or she can lift.

The growth and repair, however, cannot occur without the necessary building blocks. These are supplied by high quality nutrition. Bodybuilders require a very specialised diet. Generally speaking, bodybuilders require anything between 500-1000 calories (2000 to 4000 kilojoules) above their maintenance level of food energy while attempting to increase lean body mass.

A sub-maintenance level of food energy is combined with cardiovascular exercise to lose body fat in preparation for a contest. The ratios of food energy from carbohydrates, proteins, and fats vary depending on the goals of the bodybuilder.

Supplements can help muscle gain, although some are unproven and many are ineffective. Two supplements which have been proven to help bodybuilders gain and maintain size (without unhealthy side effects) are Creatine and L-Glutamine. Like all supplements, these only help if used in conjunction with a solid nutritional base and weight training program.

Some bodybuilders may use drugs to gain an advantage over results due to natural hypertrophy, especially in professional competitions. Although many of these substances are illegal in many countries, in professional bodybuilding the use of anabolic steroids and precursor substances such as prohormones are sometimes essential to competing in world-class competitions.

Most steroids allows the human body to be in a more anabolic state. Significant negative side-effects accompany steroid abuse, such as liver damage and negative feedback leading to a decline in the body's own testosterone production, which can cause testicular atrophy and possible infertility.