Background: Difficulties experienced by novices in clinical reasoning have been well documented in many professions, especially medicine (Boshuizen and Schmidt 1992, 2000; Elstein, Shulman and Sprafka 1978; Patel and Groen 1986; Rikers, Loyens and Schmidt 2004). These studies have shown that novice clinicians have difficulties with both knowledge and strategy in clinical reasoning tasks. Speech and language therapy students must also learn to reason clinically, yet to date there is little evidence of how they learn to do so. Aims: In this paper, we report the clinical reasoning difficulties of a group of speech and language therapy students. We make a comparison of a subgroup of these with experienced speech and language therapists' reasoning and proposes some methods and materials to aid the development of clinical reasoning in speech and language therapy students. Methods & Procedures: Student diagnostic reasoning difficulties were analysed during the assessment of unseen cases on an electronic patient database, the Patient Assessment and Training System (PATSy http://www.patsy.ac.uk) (Lum et al. 2006). Pairs of students were videoed as they completed a one hour assessment of one of three "virtual patients". One pair of experienced speech and language therapists, who were not part of the project team, also completed an assessment of one of these cases under the same conditions. Screen capture was used to record all on screen activity within PATSy web pages (i.e. mouse pointer position, hyperlink and button presses, page scrolling, browser navigation interactions and data entered); Verbal comments made by participants were analysed via a seven-level coding scheme that aimed to describe the events that occur in the process of diagnostic reasoning. Outcomes & Results: Students displayed a range of competence in making an accurate diagnosis. Diagnostically accurate students showed use of specific professional vocabulary, and a greater use of firm diagnostic statements. For the diagnostically inaccurate students, typical difficulties were a failure to interpret test results and video observations, difficulty in carrying out a sequence of tests consistent with a diagnostic reasoning path, and problems in recalling and using theoretical knowledge. Conclusions and Implications: We discuss how identification of student diagnostic reasoning difficulties can inform the design of learning materials intended to address these problems. Coding scheme is appended. (Contains 2 figures and 1 table.)