Millions of older adults are hospitalized for a critical illness each year and although they are more likely than ever to survive this illness, they commonly face significant morbidity in the form of disabilities in basic self-care activities and in mobility in the months and years afterwards. A better understanding of the underlying risk factors for disability following critical illness is greatly needed, including the effect that activity during hospitalization may have on these outcomes. Therefore, we designed the Measuring OutcomeS of Activity in Intensive Care (MOSAIC) observational study to evaluate the relationship between activity (measured more rigorously than in prior investigations) and disability, physical function, and cognitive function in survivors of critical illness 3 and 12 months after ICU discharge.