In recent years it has become clear that most animals have the ability to learn and that this often plays an important role in how individuals adjust their behaviours throughout their lives. Yet, how learning ultimately contributes to behavioural variation within populations is still poorly known. This project investigates the functioning and consequences of reward-based learning – a simple and universal mechanism by which individuals adapt their behaviour through reinforcement of successful actions – through an innovative combination of modelling, lab and field experiments. First, a theoretical framework is developed using an individual-based modelling approach to disentangle how interactions between environmental conditions, heritable behavioural traits and reward-based learning shape behavioural variation within populations within an ecological and evolutionary context. Next, the predicted eco-evolutionary interactions are validated using lab experiments with Field Crickets (Gryllus campestris), a model organism in which reward-based learning is the dominant type of learning. Finally, the generality of the predicted patterns of behavioural variation for wild populations are tested by means of a field experiment with Great Tits (Parus major), a model organism with highly developed cognitive skills. As such, the project will provide important new insights into the role of non-genetic variation, as caused by reward-based learning, for ecological and evolutionary processes.