Randomized controlled trials (RCT) are the gold standard in determining efficacy of cancer screening tests. Yet, systematic differences between RCT and the general populations eligible for screening raise concerns about the generalizability and relevance of RCT findings to guide the development and dissemination of cancer screening programs. Observational studies from clinical practice settings have documented selective uptake in screening-i.e., variation across subgroups regarding who is screened and not screened-as well as suboptimal adherence to screening recommendations, including follow-up of positive findings with subsequent imaging studies and diagnostic invasive procedures. When the effectiveness of a screening intervention varies across subgroups, and there is selective uptake and suboptimal adherence to screening in clinical practice relative to that in the RCT, the effects of screening reported in RCTs are not expected to generalize to clinical practice settings. Understanding the impacts of selective uptake and suboptimal adherence on estimates of the effectiveness of cancer screening in clinical practice will generate evidence that can be used to inform future screening recommendations and enhance shared decision-making tools.