Asthma and obesity are both chronic diseases and their prevalence has risen in recent years. Given the parallel increase, asthma and its related symptoms and phenotypes have been associated with body mass index (BMI) and obesity. Obesity may directly affect the type and severity (phenotype) of asthma by direct mechanical effects, by enhancing the immune response through related genetic mechanisms, and by sex specific influences such as hormones. Alternatively, obesity may be linked to other environmental factors such as physical activity, diet, and birth weight. These environmental influences, in combination with genetic susceptibility, may then lead to enhanced susceptibility to asthma. The features of these mechanisms are addressed.