Mast cells, traditionally regarded as effector cells of the immune system, have more recently been demonstrated to be key figures in initiating, developing and sustaining complex pathophysiological processes underlying asthma and other allergic diseases. Asthma is characterised by airway inflammation alongside a disturbance to airway physiology manifesting as variable airflow obstruction and airway hyper-responsiveness (AHR). Evidence has emerged that mast cells influence airway function by forming close intercellular relationships with different structural components of the airway wall. In asthma, mast cells are seen to localise to the airway epithelium, to mucous glands and to the airway smooth muscle (ASM). It is mast cell-ASM interaction that is most fundamental to the asthma phenotype and many mast cell mediators have been demonstrated to have important effects on ASM function. In asthma, alongside the inflammatory and physiological changes, structural changes occur to the airway wall in the form of denudation of the epithelium, goblet cell and mucous gland hyperplasia, subepithelial fibrosis, abnormal extracellular matrix (ECM) deposition, vascular proliferation and increased ASM mass. There are many ways in which mast cells can contribute to these structural changes through direct cell to cell communication and more indirectly through mediator release. Mast cells exhibit an array of diverse functions and roles and are fundamental to our current understanding of asthma pathogenesis including severe asthma. Novel targeting of mast cells and their mediators therefore should offer significant therapeutic potential in the treatment of asthma.