Mobile assessments permit an ecologically valid assessment of self-reported and objective measures of functioning and related health behaviors by capturing these processes throughout an individual’s daily life, where and when it matters. A goal is the provision of reliable, efficient, and low-cost approaches for obtaining digital biomarkers, scalable to the population. In this talk, I build on our efforts for the mobile monitoring of cognitive change, focused on the sensitive detection of within-person cognitive decline. We know from existing long-term longitudinal studies that the clinical detection of cognitive decline and dementia occurs too late for preventative and early treatment, typically detected 5 to 10 years after neurodegenerative changes have taken place. As in life course research, the causes of cognitive impairment and other health outcomes are multifactorial and relative contributions of risk factors vary substantially between individuals. We need to understand these variations better to achieve more accurate understanding of individual variation and change to develop individualized treatments. I will highlight some of the methodological advances in intensive measurement designs and analyses with examples from our research on both young and older adults.