Clinical reasoning skills underlie the decisions speech-language pathologists (SLPs) make during practice. Although clinical reasoning is included in speech-language pathology accreditation standards in the United States, there is limited research on preservice SLPs’ development or use of clinical reasoning skills. Because clinical reasoning skills specific to augmentative and alternative communication (AAC) service provision have not been identified or reported in depth, this qualitative study focused on two cases of children with developmental disabilities who used AAC systems. Eight novice (i.e. preservice) SLPs and eight expert SLPs completed think-aloud tasks while they developed intervention plans, which made their clinical reasoning skills observable. Responses were transcribed for qualitative analysis. Six clinical reasoning skills were identified: summarizing, interpreting, hypothesizing, rationalizing, comparing and deferring. Expert and novices used four of these clinical reasoning skills similarly; however, there were differences among the remaining two skills. Novices deferred more frequently during the tasks and were limited in their ability to make comparisons to the fictional cases as compared to the experts. The expert-novice gap in clinical reasoning presents implications for teaching preservice SLPs in the classroom and the clinic.