Preventable environmental causes of cancer, including tobacco smoke and other carcinogens in the diet, workplace, and ambient environment are responsible for the vast majority of human cancers. This paper reviews recent molecular epidemiologic studies that have focused on environmental carcinogenesis and environment-host interactions. Biomarkers such as carcinogen-DNA and carcinogen-protein adducts, mutations in reporter or target genes (e.g., HPRT, GPA, ras, p53), or genetic or acquired susceptibility factors (e.g., polymorphisms in the P450 or glutathione-S-transferase genes and serum levels of antioxidants) have shown significant potential in prevention. They should be useful in early identification of at risk individuals and in designing and monitoring interventions (smoking cessation, exposure reduction, and chemoprevention).