The project aims to describe how people are similar and how they differ in their ways of speaking and understanding spoken language and to explain how these differences arise. Speakers of a common language must share substantial parts of their grammatical and lexical knowledge; otherwise they would not understand each other. It is reasonable to assume that the shared linguistic knowledge is represented and accessed in similar ways by all speakers. Linguistic and psycholinguistic research has typically been directed at uncovering these commonalties between people. However, in everyday conversations we can observe marked differences in the way people express themselves, for instance in their speech rate and fluency, word choice and utterance complexity. There are also striking differences in people s ability to understand spoken language, especially under unfavorable conditions. In psycholinguistic experiments, we also find that participants differ in their performance, for instance in their processing speed or accuracy, or in the sizes of experimental effects. The first aim of the project is to describe how healthy adults differ in the way they comprehend and produce spoken utterances. Although differences among adult speakers and listeners are noticeable in everyday life, experimental psycholinguists have largely ignored them, focusing instead on the average speaker/listener. Moreover, most experimental research has been carried out with university students as participants. Given that student samples are likely to be more homogeneous in their cognitive and linguistic abilities than the general population, very little is known about the ways people differ in producing and understanding utterances. Our first aim then is to determine in which tasks and situations adult speakers and listeners differ substantially in their performance and where performance differences are minimal. The second aim of the project is to explain how similarities and differences between speakers and listeners arise. It is likely that performance differences in linguistic tasks such as describing a scene or understanding complex sentences are due to the joint effects of several general cognitive and specific linguistic factors. We aim to determine what these factors are and how much of the observed variability each of them explains.