Performance-enhancing substances, also known as performance-enhancing drugs (PED), are substances that are used to improve any form of activity performance in humans. A well-known example involves doping in sport, where banned physical performance–enhancing drugs are used by athletes andbodybuilders. Athletic performance-enhancing substances are sometimes referred to as ergogenic aids. Cognitive performance-enhancing drugs, commonly called nootropics, are sometimes used by students to improve academic performance. Performance-enhancing substances are also used by military personnel to enhance combat performance.

The use of performance-enhancing drugs spans the categories of legitimate use and substance abuse. The classifications of substances as performance-enhancing substances are not entirely clear-cut and objective. As in other types of categorization, certain prototype performance enhancers are universally classified as such (like anabolic steroids), whereas other substances (like vitamins and protein supplements) are virtually never classified as performance enhancers despite their effects on performance. As is usual with categorization, there are borderline cases; caffeine, for example, is considered a performance enhancer by some but not others.