Illusory conjunctions were originall y offered as part of the convergent evidence that supported Feature Integration Theory (FIT) (Treisman, 1988i Treisman & Schmidt, 1982). According to FIT, illusory conjunctions arise from a lack of focal attention. An extended version of Prinzmetal and Keysar's ( 1989) functional explanation of illusory conjunctions is offered as an alternative to FIT's account of illusory conjunctions. This functional explanation rests on the hypothesis that illusory conjunctions are the result of an adaptive response to very brief viewing times in the presence of visual location information that constrains both feature integration and visual attention. Two experiments were conducted to test predictions arising from this hypothesis. In these experiments the effects of a black square outline on illusory conjunction and non conjunction error rates were examined. The results of the experiments indicate that the critical factor in eliciting illusory conjunctions is the presence of constraining visual information. It is also demonstrated that such constraints not only lead to illusory conjunctions, but also facilitate the accurate detection of objects. The apparent conflict in the claim that constraints lead to illusory conjunctions, which are errors, and at the same time facilitate accuracy can be resolved by the functional explanation. Some suggestions are made as to which visual stimuli might operate as constraints on feature integration and visual attention.