Bronchial reactivity is a physiological property of healthy airways to develop a moderate airway obstruction in response to various non-specific stimuli, which is altered in several pulmonary diseases. The active effector of airway reactivity is airway smooth muscle (ASM). The contractile status of airway smooth muscle is under the control of many extracellular messengers acting on specific membrane receptors. Binding of the contractile messengers to their specific membrane receptors increases cytosolic Ca2+ concentration ([Ca2+]i). The shape of the resulting calcium signal is sensed by the contractile apparatus and hence determines the pattern of the contractile response. Agonists can also modify the sensitivity of the contractile apparatus to calcium, via phosphorylation and dephosphorylation of a network of regulatory proteins. These mechanisms can be altered in several respiratory diseases such and COPD, asthma, or exposure to air pollutants, leading to hyperreactivity, which can be pharmacologically controlled by drugs acting on the mechanisms of ASM contraction.