Background: A stepwise, multidisciplinary and multicomponent intervention (called STA OP!) was implemented in Dutch nursing home units, which included a comprehensive multidisciplinary team training. A cluster-ran-domised controlled trial showed that the intervention reduced symptoms of pain and challenging behaviour. Objective(s): To describe the experiences around the implementation of the intervention; to examine the extent to which the STA OP! intervention was delivered and implemented as intended (at the level of the team, and the individual resident/professional); and to understand factors influencing the implementation process. Methods: A process evaluation was performed using a mixed-methods design encompassing several data sources. Quantitative data (i.e. from the written evaluations by healthcare professionals, management, and the research database) were analysed using descriptive statistics. Qualitative data (i.e. semi-structured inter-views, notes, completed intervention forms, and written evaluations) were analysed according to the princi-ples of thematic analysis. The implementation process and the influencing factors were categorised according to the i) organisational level, ii) the team level, and iii) the level of the individual resident/professional. Results: In total, 39.2% of the residents with pain and/or challenging behaviour were treated following the stepwise approach of the STA OP! intervention. The training manual and forms used were found to be relevant and feasible. Factors inhibiting the implementation process at the i) organisational level concerned instability of the organisation and the team (e.g. involvement in multiple projects/new innovations, staff turnover/absence of essential disciplines, and/or high workload). At the team level (ii), we found that pres-ence of a person with a motivational leadership style facilitated the implementation. Also, interdisciplinary cooperation through the design/setting of the multidisciplinary training, securing the intervention by use of clear agreements, and written reporting or transfers facilitated implementation. At the individual level (iii), perceived value of the stepwise working method, and enhanced awareness facilitated the implementation. Conclusion: Although the intervention was not implemented as planned, the intervention empowered healthcare professionals and increased their awareness of the signals of pain and challenging behaviour. Future implementation of the intervention should start on units with a motivational leader, and specific features of the organisation and the team should be considered to facilitate implementation, e.g. stability, support, and shared focus to change.