This paper provides more insight into the assumption of human capital theory that the productivity of job-related training is driven by the improvement of workers' skills. We analyze the extent to which training and informal learning on the job are related to employee skill development and consider the heterogeneity of this relationship with respect to workers' skill mismatch at job entry. Using data from the 2014 European Skills and Jobs Survey, we find – as assumed by human capital theory – that employees who participated in training or informal learning show greater improvement of their skills than those who did not. The contribution of informal learning to employee skill development appears to be larger than that of training participation. Nevertheless, both forms of learning are shown to be complementary. This complementarity between training and informal learning is related to a significant additional improvement of workers' skills. The skill development of workers who were initially underskilled for their job seems to benefit the most from both training and informal learning, whereas the skill development of those who were initially overskilled benefits the least. Work-related learning investments in the latter group seem to be more functional in offsetting skill depreciation than in fostering skill accumulation.