<p>For adolescents and young adults (AYA) with any health condition, it is important that they learn to manage their condition and healthcare as they transition into adulthood. For AYA with childhood-onset long-term conditions, this is also fundamental for a successful transition from child- to adult-centred services, as this period is associated with a decline in important health behaviours, such as medication adherence, which in turn are associated with poorer clinical outcomes and increased mortality. Current evidence suggests that, even though AYA are at higher risk for non-adherence than other age groups, existing interventions are less likely to be effective, or may bring about more modest benefits, for AYA compared with younger children and older adults. There is still a need for novel, innovative approaches to medication adherence that can help better meet the unique needs of AYA groups. We suggest that developmentally informed and developmentally tailored approaches may offer a promising avenue to achieve this. The most widely reported AYA adherence issues are deeply intertwined with the different stages of AYA biopsychosocial development and, therefore, AYA development can be understood as a common thread underlying AYA adherence issues. Ensuring that AYA adherence-promoting interventions are relative to an ongoing developmental assessment is crucial, not only to better meet AYA needs as they gradually prepare for their transfer to adult care, but also to continue to do so in the often forgotten third phase of transitional care (i.e. following transfer) well into their late adolescence and young adulthood.</p>