Nanowire gas sensors have many applications in health-care, safety, environmental monitoring etc., yet the major problem that current research faces is a lack of understanding of the reactions that take place between the sensing film and the gas species and hence a substantial lack of selectivity of tindioxide nanowire sensing devices. The book will provide an introduction to nanowire gas sensor modelling and will in particular present a mathematical model for tindioxide nanowire sensors. The surface reactions are described by parameter-dependent ordinary differential equations (ODEs) that give the net exchange of electrons between the gas and the tindioxide nanowire. The net exchange of electrons is then included in the nonlinear Poisson-Boltzmann equation for the computation of the electric potential. From the electric potential we obtain the concentrations of holes and electrons via Boltzmann distributions and finally the graded channel approximation returns the current I that can be compared to the measurements. The book provides the reader with an overview about developing a simulation model for gas sensors.