In order to enrol children at potential risk for the outcomes of interest, and avoid "healthier" participants with short PICU stays, we considered the following: the patient should have: a) a minimum age (when one is expected to be ambulatory/mobilizing), and b) a threshold severity of illness. In order to understand how different subsets of PICU populations are affected by critical illness, we chose not to restrict the selection of patients to a specific diagnosis, nor did we exclude patients with underlying functional or cognitive abnormalities at baseline. This study will evaluate the epidemiology of recovery in a general critically ill pediatric population over time.