Research using interpreters who work with signed languages can aid us in understanding the cognitive processes of interpretation in general. Using American Sign Language (ASL) as an example, the nature of signed languages is outlined first. Then the difference between signed languages and manual codes for spoken languages is delineated, and it is argued that these two manners of communicating through the visual channel offer a unique research opportunity. Finally, an example from recent research is used to demonstrate how comparisons between spoken-language interpreters and signed-language interpreters can be used to test hypotheses regarding interpretation.