The assessment of communication disorders is, by necessity, a behavioural description process. Standardized measures provide quantitative data that contribute to the identification of communication disorders, but test scores and normative comparisons are insufficient for describing the effects of impairment on performance in communicative settings. The importance of ecological validity in tests for communication disorders, the need for systematic observation and the potential biases in testing diverse populations are considered within the framework of identification and differential diagnosis of speech and language disorders. There are also communicative disorders that are clinically recognized, but which cannot be reliably identified following standardized testing and systematic observation in communicative settings. For these disorders, there is a continuing need for research based on behavioural description to develop a theoretical understanding of the disorder that can guide reliable assessment, identification and intervention.