This study investigated the use of schema-specific diagrams in probability problem solving. Graduate students enrolled in an introductory probability and statistics course solved four probability problems, with and without instructor-provided diagram hints. Participants solutions were examined and coded for correctness, use of provided diagrams, and use of student-generated external visual representations. Results show that provided diagram hints helped low-ability students on all but the most difficult problem, while high-ability students were aided by diagrams on the most difficult problem. Implications for the use of diagrams in the development of problem solving proficiency are discussed.