The number of overweight and obese individuals in the population has increased dramatically in the past few decades, and the rising prevalence of obesity is a major public health concern. Growing evidence has accrued for obesity as a risk factor for venous thrombosis. The risk of venous thrombosis increases in a dose-dependent manner with increasing body mass index and is also associated with the majority of other anthropometric measures of overweight and obesity, such as waist circumference, hip circumference, and waist-to-hip ratio. An increased relative risk of both unprovoked and provoked venous thrombosis has been shown in obese compared with normal-weight subjects. However, encountering obesity as a causal factor for venous thrombosis is problematic due to the ill-defined concept of obesity. In this review, we will examine the current epidemiological evidence for an association between obesity and venous thrombosis. We will comment on the problem of causal interpretation of obesity per se and discuss how individual components that define obesity can serve as potential biological mechanisms for the observed association between obesity and venous thrombosis.