this thesis aimed to assess the effect of multimorbidity (mmb) on colorectal cancer (crc) patients’ survival and explored whether sociodemographic and health system factors affected this relationship or not. this thesis also describes the complexity of crc patients through the description of condition combinations and related health services use. methods: a retrospective cohort study was conducted using administrative data from the institute for clinical evaluative sciences (ices). the population was adult ontario residents who were diagnosed with crc between 2003 and 2013 and were followed until march 31, 2018. the exposure of interest, mmb, was defined as having one or more of 17 common conditions in addition to crc and categorized (1, 2, 3, 4 or more). conditions diagnosed prior to or within 30 days of crc diagnosis were included. survival analyses were performed using cox proportional hazards regression to assess the association between mmb and crc patients’ survival. to investigate additional factors associated with crc patients’ survival, cox models were adjusted for sociodemographic (age, sex, income, ethnicity, and rurality) and health system factors (primary care models (pcms), continuity of care (coc), and primary care (pc) visits) as well as cancer stage.