Screening for cancer or cancer risk is well-established in high-income countries. This article considers ethical aspects of cancer screening. Ethical evaluation of screening depends on a contested evidence base, interacts with people's fear of cancer, and their enthusiasm for technology in general and screening in particular. Cancer screening is both a clinical and a public health activity, and so the often-conflicting frameworks from both clinical ethics and public health ethics are relevant to its evaluation. Cancer screening is an intrusion by health services into the lives of well individuals and so requires strong justification. Cancer screening can and should prevent harms to physical health, but its ability to do so is contingent on many factors and finely balanced; screening can also affect psychological wellbeing. When communicating about screening programs, care must be taken to support rather than undermine the autonomy of people considering participation. The benefit offered by cancer screening programs should be large enough to justify the opportunity costs of screening and the consequent cascade of intervention. Treatment should be offered in a way that avoids creating financial strain for individuals. Other relevant ethical issues include equity of opportunity and outcome in screening and accountability to communities. It is not clear how population-level and individual-level outcomes and interests in cancer screening should be balanced; future work should focus on resolving these difficult issues.