The Cancer Barometers are regular surveys conducted among the French public with the aim of analyzing changes in their knowledge, perceptions, and lifestyle habits regarding cancer and its main risk factors. This fourth edition, which presents the results of the 2021 survey conducted by the National Cancer Institute in collaboration with Santé publique France, serves as a tool for reflection on future practices and policies, opening new avenues for research in the field of cancer control. It measures changes in knowledge, perceptions, attitudes, and lifestyle habits related to cancer and its main risk factors. It is intended to help adapt government action to changing perceptions and behaviors regarding this disease. The 2021 Cancer Barometer was supported by a strategic committee, a support and development committee, and an editorial committee. The development of the questionnaire was overseen throughout 2020. The pilot study and the field survey were conducted in 2021—following the first lockdown and at the time the ten-year strategy was launched—by Ipsos teams. It includes the following chapters: information on cancer and its risk factors; tobacco and cancer; alcohol and cancer; ultraviolet radiation, natural or artificial; nutrition and cancer; e-cigarettes; perceptions and uptake of screening and HPV vaccination.