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Rare until recently, field-experimental studies of interspecific competition now number well over 150. Competition was found in 90% of the studies and 76% of their species, indicating its pervasive importance in ecological systems. Exploitative competition and interference competition were apparent mechanisms about equally often. Few experiments showed year-to-year variation in the existence of competition, though more did in its intensity; many were not long-term. The Hairston-Slobodkin-Smith hypothesis concerning variation in the importance of competition between trophic levels was strongly supported for terrestrial and freshwater systems. In particular, producers, and granivores, nectarivores, carnivores, and scavengers taken together, showed more competition than did phytophagous herbivores and filter feeders. In marine systems, virtually no trend was detectable one way or the other. Large heterotrophs competed more than small ones in most comparisons, and other properties possibly deterring predation, such as stinging behavior, seemed also characteristic of species competing frequently. Among terrestrial plants and certain terrestrial animals but not all, experiments carried out in enclosures were more likely to show competition than unenclosed experiments. A greater ecological overlap implied a greater tendency to compete, as determined experimentally, when niche dimensions were food type or microhabitat; the opposite was true for macrohabitat. A substantial number of studies showed asymmetry in their species' response to competition; larger species were significantly more often superior than smaller ones, though a variety of other apparent reasons for asymmetry also existed. The integration of competition theory into field experimentation has only just begun.