Childhood vaccination is a powerful tool for reducing morbidity and premature deaths, and vaccines are usually provided for free. Despite this, several low- and middle-income countries are lagging far behind in terms of their vaccination coverage. This article uses insights from behavioural economics to shed light on the mechanisms at work when people make decisions about childhood vaccination, thus explaining why demand for vaccination may be low. Some of the factors highlighted are the immediacy effect, loss aversion, overestimating and overweighting of small probabilities, and social preferences and trust. We conclude that these factors have important implications for how incentive mechanisms for vaccination should be designed, how vaccination decisions ought to be framed, who information campaigns should be conducted, as well as for the need to build trust in vaccine providers and health systems more generally.