Recent theories of concepts have raised the issue of how people combine simple concepts (like engine and repair) to form complex concepts (like engine repair). This article approaches this issue by asking how people comprehend modified noun phrases of this sort. One explanation of how complex concepts are understood (the feature weighting model) provides a simple mechanism in which the primary feature of the modifying concept is made more salient in the modified concept. Another explanation focuses on how world knowledge directs the combination process. The two explanations are compared in their ability to account for the interpretation of various kinds of noun phrases. Two experiments are reported which evaluate the feature weighting model's predictions for adjective-noun phrases. These contrasts suggest that the combination process does require reference to world knowledge. The consequences of accepting such an account are discussed.