Handwashing with soap (HWWS) is effective in reducing the transmission of important infectious diseases such as diarrhoea and acute respiratory infections (ARIs). Diarrhoea and ARIs remain two of the leading causes of child morbidity and mortality, with especially high rates among children in humanitarian emergency settings. Rates are highest among children under-five; however older children (children aged 5-14) also bear a large burden of these diseases. Older children begin to take responsibility for their own behaviour so promoting HWWS among this age group can achieve a significant public health impact. What works in HWWS interventions targeting older children in emergency settings, however, is unclear. This PhD thesis aims to understand the determinants of handwashing behaviour among older children in emergency settings and identify potentially effective approaches to HWWS interventions targeting this population. The thesis synthesises research from a systematic review assessing the effectiveness of different behaviour change techniques (BCTs) employed across hand hygiene interventions for older children, a study exploring the perceived determinants of older children’s handwashing behaviour in an internally displaced persons (IDP) camp in Northern Iraq, a trial of a novel HWWS intervention employing motivational drivers, also in this IDP camp, and a qualitative study exploring NGO practitioner’s perspectives on the challenges and solutions to HWWS interventions targeting older children. Findings suggest HWWS interventions targeting older children in humanitarian emergencies should be fun, interactive, and as low-resource as possible so that they are simple, quick to implement, and easy to replicate. Using ‘positive’ motivational drivers, including play and nurture, creating a physically enabling environment that includes environmental cues, and leveraging social norms were all approaches found to be potentially effective. Interventions should build on a basic package of BCTs which ensure handwashing facilities and materials are available, if not already, and older children understand how to perform HWWS. Formative research should be undertaken to establish existing levels of health-related knowledge of handwashing and determine if health-based messaging is included. If so, this should be carefully designed to engage older children and create a tangible link between HWWS and health. To aid improvements in intervention effectiveness, the NGO sector must make HWWS promotion for older children in emergency settings an organisational priority. New interventions should be evaluated as rigorously as possible within the context to build the evidence base and approaches that prove successful should be standardised and shared across the sector.