How students develop vocational knowledge is a rather under-researched topic in the context of vocational education and training. Vocational knowledge is perceived as the kind of knowledge required to perform in occupational practice. From an activity-theoretical approach to learning, supplemented with ideas borrowed from inferentialism, this article explores how students develop vocational knowledge in terms of a cognitive activity of contextualising. A qualitative in-depth study is presented, which explores students' cognitive processes during professional performance. Hospitality students and culinary students were interviewed and asked to articulate the process of contextualising during their work in a sandwich bar. A detailed description of the characteristics of contextualising is presented, and the process is illustrated with examples from the data.