Process evaluation (PE) is used for the in-depth evaluation of the implementation process of health promotion programmes. The aim of the current paper was to present the PE design and tools used in the ToyBox-intervention. The PE design was based on a three-step approach, including the identification of ToyBox-specific PE elements (step 1), the development of PE tools and harmonization of procedures (step 2), and the implementation of PE using standardized protocol and tools across the intervention countries (step 3). Specifically, to evaluate the implementation of the intervention, teachers' monthly logbooks were recorded (dose delivered, fidelity, dose received); post-intervention questionnaires were completed by parents/caregivers and teachers (dose received); participation and attrition rates were recorded (recruitment, reach); and audit questionnaires and retrospective information on weather conditions were collected (physical and social environment within which the intervention was implemented). Regarding the teachers' training sessions, the researchers who performed the trainings completed evaluation forms and documented teachers' attendance after each training (dose delivered, fidelity, dose received) and teachers completed evaluation forms after each training (dose received). The PE performed in the ToyBox-intervention may contribute in the evaluation of its effectiveness, guide the revision of the intervention material and provide insights for future health promotion programmes and public health policy.