Obesity has been associated with an increased prevalence of asthma and poorer control of this disease. However, the mechanisms by which obesity can influence airway function and make asthma more difficult to control remain uncertain. The physiological changes associated with obesity can contribute to respiratory symptoms and these should be differentiated from those caused by asthma. Obesity can possibly influence the development of asthma through genetic, developmental, hormonal, neurogenic or mechanical influences. Breathing at low lung volumes and changes in the pattern of breathing in obese subjects may alter airway smooth muscle plasticity and airway function. The release by adipocytes of various cytokines and mediators such as Interleukin-6, TNF-alpha, eotaxin, and leptin, and the reduction of anti-inflammatory adipokines in obese subjects may possibly contribute to the development or increased clinical expression of asthma in promoting airway inflammation. Reduced asthma control and impaired response to asthma therapy have been reported in obese patients. Obesity-related co-morbidities such as Sleep Apnea and Gastro-esophageal reflux may also contribute to this poor control. Weight loss improves asthma control and reduces medication needs. Research is needed to better define the optimal management of obese asthmatic patients.