The aim of the CCSS is to better understand the long-term effects of childhood cancer and its treatment, as well as determine the factors that increase the risk for adverse outcomes. Participants includes individuals recruited from 31 clinical centres across North America who survived five or more years after a cancer diagnosis during childhood or adolescence (< 21 years) and their siblings, who serve as a comparison group. The original cohort included over 14,000 people who had been diagnosed with cancer between 1970 and 1987, completing baseline assessment in 1994. The expansion cohort was added in 2008, including a further 11,000 people who were diagnosed with cancer between 1987 and 1999. Over 5,000 siblings are included in the study from baseline data collection in 1994. Participants have been followed up for the main study up to 8 times, with the most recent follow up in 2022. There have also been several ancillary studies, in addition to the main questionnaire and biological samples collected.