Speech-language pathologists routinely use picture description tasks to assess expository discourse in clients with disorders such as aphasia and dementia. One picture description task – the Cookie Theft picture from the Boston Diagnostic Aphasia Examination – has come to dominate clinical settings more than any other task. This article examines why this particular picture description task has proven to be so successful in assessing expository discourse in clients with language and cognitive disorders. Using data from the University of Pittsburgh Alzheimer and Related Dementias Study, recurrent cognitive-linguistic impairments in the Cookie Theft picture descriptions of clients with Alzheimer’s dementia are explored. These impairments are mostly pragmatic in nature. It is argued that the sensitivity of the Cookie Theft picture description task to these impairments makes it an ideal assessment tool for any investigation which aims to identify pragmatic markers of neurodegenerative diseases such as the dementias.