Lumped-parameter models of lung function assign the constitutive properties of the lung to distinct and separate structural entities. Such models are typically described by sets of coupled ordinary differential equations. The parameters of these equations correspond to specific physical attributes of the lung such as airway resistance or tissue elastance. The fundamental unit of a lumped-parameter model is the compartment, which corresponds to some space within which the time-varying quantity of interest is uniformly distributed. The parameters of a lumped parameter model can often be related to fundamental physical processes, such as viscous flow of gas through airways or the elastic recoil generated when the lungs are inflated. The simplest lumped parameter model of the lung consists of a single elastic compartment served by a single flow-resistive airway. Additional realism can be added to this model by making its constitutive properties nonlinear or by connecting it to additional compartments. If the lung behaves sufficiently like a linear dynamic system, its behavior can be encapsulated in the complex function of frequency known as mechanical input impedance, which itself can be described by lumped-parameter models.