Follicular lymphoma (FL) accounts for one-third of incident non-Hodgkin lymphomas in Western countries, but its etiology is largely unexplained. I performed systematic reviews and meta-analyses, and used a population-based family case-control study to investigate the relationship between lifestyle, environmental and occupational risk factors of FL. Meta-estimates from my random-effect models showed a non-significant association with smoking, heterogeneous results for alcohol, modest increased risk with obesity, and positive associations with exposure to polychlorinated biphenyls (PCBs), chlorinated solvents and dichlorodiphenyldichloroethylene (DDT, a pesticide). The population-based case-control study included 770 FL cases and 490 family controls (siblings, partners). Participants completed a lifetime residential and work calendar and health, lifestyle, and diet questionnaires. I used unconditional logistic regression to examine associations with FL risk, including group-based trajectory modeling to examine associations with body shape and outdoor hours over the life course. I identified deaths using record linkage and applied Cox proportional regression to estimate hazard ratios for all-cause and FL-specific mortality. I observed a positive association between smoking and FL risk and mortality. Associations with recent alcohol intake and FL were null. Being obese 5 years prior to enrolment and higher body mass index 5 years prior to enrolment was associated with a modest increased FL risk, but there was no association with body shape trajectory. Body size was not associated with mortality. I observed an elevated FL risk with consumption of oily fish, but no association with mortality. I found no significant association between occupational exposure to pesticides, or extremely-low frequency magnetic fields, and FL risk. For sun exposure, I observed an inverse association with high cumulative outdoor hours and high outdoor hour maintainers over the life course, and FL risk. Policies on tobacco control, maintaining body weight within normal range, and safe use of solvents and pesticides are crucial in reducing the burden from FL. Sun exposure is not recommended as a cancer control policy, but the association may inform research on targeted therapies for this malignancy. My key findings have advanced our understanding of FL etiology and help guide risk reduction strategies and future research.