This article describes the method used to estimate the proportion and number of cancers attributable to lifestyle and environmental risk factors among adults in France in 2015. Thirteen factors were studied: tobacco, alcohol, diet, overweight and obesity, insufficient physical activity, exogenous hormones, suboptimal duration of breastfeeding, infections, ionizing radiation, air pollution, solar radiation (UV), occupational exposures, and exposure to chemicals in the general population (arsenic in water and benzene in indoor air). Exposure data representative of the French population around 2005 and estimates of the cancer risk associated with these exposures were used to estimate the attributable fraction (AF) for each factor in the population aged 30 and older, by site and by sex, assuming a latency period of 10 years between exposure and cancer risk. The estimated number of new cancer cases in 2015 was multiplied by the AAs to obtain an estimate of the number of cancers attributable to each factor. The project involved more than 80 collaborators from leading French research and public health institutions, thereby ensuring institutional consensus on the data and methods used, as well as on the results and their interpretation. The estimation of the FA is based on strong assumptions (a 10-year latency period, independence among the risk factors studied) and is limited by uncertainties regarding risk estimates and population exposure to risk factors. Nevertheless, it constitutes a valuable tool for quantifying the number of preventable cancers in France, which is essential for informing the public and implementing future targeted prevention measures.