Recursion is a fundamental concept in Computer Science. A student's knowledge of recursion can be termed their mental model of recursion. A student's mental model is viable if it allows them to accurately and consistently represent the mechanics of recursion. Non-viable mental models are constructed if students have misconceptions about the mechanisms of recursion or have misconceptions about concepts fundamental to recursion. This paper presents a study of the mental models of recursion that first year students at the University of the Witwatersrand have constructed in 2000, 2001 and 2002. It was found that while the majority of students constructed the viable copies model, many non-viable models such as the looping, active, step, magic, return-value and various odd models were also constructed. Identifying the models that students have can allow lecturers to target individual students' specific problems and analysis of the models can provide insight into learning.