Around 50 million people globally have dementia and this number is set to triple by 2050. Given the significant impact of dementia, risk reduction is a key pillar in the WHO Global action plan on the public health response to dementia. Combining physical activity (PA) and cognitive activity (CA) is a promising risk reduction strategy through a synergistic impact of PA and CA on brain plasticity. Despite the evidence supporting the benefits of simultaneous PA and CA, the precise relationship between PA, CA and cognitive function in people with different levels of cardio-respiratory fitness (CRF) is unclear. We will address this knowledge gap by investigating the dose-response relationship between simultaneous PA and CA and cognitive function in older adults with different levels of CRF in a controlled setting. We will then translate these findings to real-life settings by developing an innovative intervention adapted to different CRF levels using a structured participatory co-creation process with stakeholders and end-users. Finally, we will examine feasibility and effects of the program in a randomized controlled mixed-method efficacy study. By combining new knowledge, a participatory approach and methodologically strong intervention evaluations, the CONSCIOUS project will make a significant contribution to the state of the art in this emerging field, with the potential to positively impact public health outcomes and quality of life in the aging population.