Cancer is increasing worldwide. Patterns of cancer are also changing. The evidence is summarised in the 2018 World Cancer Research Fund/American Institute for Cancer Research report Diet, Nutrition, Physical Activity and Cancer: a Global Perspective. The plasticity of cancer patterns implicates environmental factors as determinants of cancer, and nutrition influences key cellular and molecular processes that characterise cancer. Epidemiology identifies associations between aspects of diet, nutrition, and physical activity with one or more cancers; there is evidence for plausible mechanisms that imply that these are causal. Some nutritional exposures (alcohol and processed meat) are likely causal factors, but no singular factor protects against cancer (except dietary fibre for colorectal cancer). Cancer protection mainly derives from a systemic metabolic environment that promotes healthy cell replication and tissue integrity. Such a nutritional state reflects avoiding excess adiposity through healthy dietary patterns rich in plant foods (legumes, wholegrains, pulses, vegetables and fruits), with modest meat, fish and dairy, low in alcohol and salt preserved foods, and an active way of life, avoiding sedentary behaviours. Less is known about the impact of nutritional interventions in people with a diagnosis of cancer, but nutrition including adiposity and physical activity predict breast cancer outcome. Promoting healthy ways of life requires public information and education, but alone these do not generate change; a socio-political and cultural environment that is conducive to adopting healthy behaviours is needed. Uncertainties in the evidence offer promising directions for future research, but sufficient is known to act as a basis for public policy and clinical practice.