Overeducation has usually been considered a phenomenon brought about during integration into the labour market. There is no single explanation, but rather several factors that are associated with overeducation. We analyse overeducation among graduates in Catalonia four years after finishing their degrees. The analysis is based on the self-assessment made by workers in surveys conducted by AQU (Catalan University Quality Assurance Agency) between 2008 and 2014 and we use logit and probit statistical models to determine the probability of a graduate being overeducated, depending on a wide range of economic, sociological, technological and academic variables. We use the Heckman methodology in the analysis. This study corroborates the results of previous studies on the relationship between wages, job satisfaction and overeducation. In addition, the results show, firstly, differences in levels of overeducation between different fields of study, most notably between 'Humanities and Arts' and 'Health Sciences'. Secondly, the results reveal the impact of the economic cycle on overeducation. Thirdly, the variables used in the statistical model exhibit stable behaviour and, as a result, they provide an explanation for overeducation as a structural phenomenon, regardless of the economic situation. Furthermore, other variables show a significant relationship with signalling theory and career mobility theory, which both explain the overeducation phenomenon. We especially emphasise the role of professional career development during undergraduate studies. Additionally, we found that the family socio-economic environment is relevant in explaining overeducation and, lastly, technological factors and aspects of the graduates' work environment also contribute to explaining the phenomenon.