Neurotrophins are a family of four proteins that support the growth and survival of neurons. Their secretion increases during brain development, when new neurons are being formed and existing ones are branching to assemble complex neuronal circuits. In addition to their role in promoting neuron growth and development, neurotrophins are also a product of neuronal activity. Neurotrophins are also responsible for the maintenance of peripheral sensory neurons, including those in the lungs. Airway innervation is responsible for many aspects of lung function including the regulation of airway smooth muscle tone, mucus secretion, and reactivity; therefore, a physiological expression of neurotrophins in the lungs is required for normal lung function.