There are two areas in the study of language and communication disorders where the lexical/functional distinction has played an important role: agrammatic aphasia and Specific Language Impairment (SLI).

With respect to agrammatic aphasia, Sloan Berndt (1990: xxv) writes: ‘In the context of a fairly well-preserved ability to communicate meanings, the agrammatic patient appears to have lost some very essential part of the language system.’ Patients speak very haltingly and with lots of pauses; there is less variation in grammatical forms, and patients tend to omit ‘grammatical function words and bound grammatical markers’.

SLI is a developmental disorder, in which proficiency scores for first language development fall outside the normal range, although other cognitive capacities do not. In the case of SLI the development of specific functional categories will lag systematically behind what would be expected in typically developing children, depending on the language involved.

For both types of disorders it is claimed that functional categories are particularly affected. However, in different studies different functional categories are highlighted, and a cross-linguistic evaluation of the evidence is needed. I will first discuss agrammatic aphasia in more general terms, before surveying different studies with respect to evidence about special problems in the production of functional categories (leaving aside the much scarcer but equally important evidence about comprehension). In the second part of the chapter I turn to the phenomenon of SLI and then survey a number of studies in this domain, with a cross-linguistic perspective.