Survival after colorectal cancer (CRC) diagnosis strongly depends on local tumor extent, lymph node involvement and the presence of distant metastases. However, there remains great inter-patient variability regarding treatment outcome. A combination of biochemical factors, histopathological features, genomic profile, environmental factors and other clinical factors are likely to influence prognosis and treatment effect, independent from tumor stage, but it is still unclear which, how, and to what extent these factors can influence tumor recurrence and mortality in both early stage (I-III) and late stage (IV) CRC, small bowel cancer and anal cancer. Although the results from prospective clinical trials will remain the backbone of evidence based medicine, this concerns a highly selected patient population since the large majority (85%-95%) of cancer patients do not participate in clinical trials for various reasons. It is unlikely that trial participation will significantly improve in the near future. This fact has the following implications: 1) It is highly desirable to validate the results from trials in the general patient population. However, this is complicated by the fact that the documentation of patients treated in general practice (i.e. outside the scope of clinical trials) is largely insufficient to provide comparable patient cohorts in terms of prognostic characteristics and treatment parameters. 2) There is an increased availability of novel technologies that provide molecular markers with potential prognostic and/or predictive value. To test the clinical value of these markers large numbers of patients are required which greatly exceeds the number of patients who consent to participate in prospective clinical trials. 3) as a result of rapid technical developments, a range of new minimally invasive treatment options are entering the market. These interventions have the potential to be of great benefit for patients in terms of improved local control, higher probability of complete tumor removal, less damage to surrounding tissue, faster recovery and less short and long term side effects. Still, the interventions will have to prove their effectiveness, safety and superiority (or non-inferiority) to standard cancer treatments on a patient level. A prospective observational cohort study has the great opportunity to fill this gap.