Just-in-time adaptive interventions (JITAIs) are interventions that adapt based on an individual’s situation to provide support that is delivered “at the moment and in the context that the person needs it most and is most likely to be receptive”. We have recently developed a smartphone-delivered JITAI targeting physical activity and sedentary behaviour in older adults. Our JITAI design was underpinned by behaviour change theory and guided by recommendations from the literature. However, throughout the development process we encountered several challenges that required tough decisions and levels of compromise. To advance the state of the art, it would be beneficial for researchers to collaborate and share their approaches to some of the tensions that arise during JITAI design so that we may progress the field. We would like to share our own experiences with JITAI design, along with feedback from our participants on what aspects worked and didn’t work for them. We would then invite participants to share their experiences and/or views.