A discrete model of the integer quantum Hall effect is analysed via its associated C*-algebra. The relationship with the usual continuous models is established by viewing the observable algebras of each as both twisted group C*-algebras and twisted cross products. A Fredholm module for the discrete model is presented, and its Chern character is calculated. § 1. IntroductionThe discovery of the integer quantum Hall effect has prompted a wealth of theoretical speculation about the origin of the spectacular accuracy with which the Hall conductance is quantized. This paper presents a simple lattice model of the quantum Hall effect that generates much of the information arising from more complex models. This lattice model of the quantum Hall effect is often used as the discrete analogue of the Landau Hamiltonian in the physics literature, and the analysis of the model often requires restricting to rational values of the magnetic flux. It is here extended and recast to fit into the C*-algebraic framework, a development that allows (in § 3) the Hall conductance to be calculated for all real values of flux. The analysis of the expression for the conductance makes its stability with respect to small changes in magnetic field evident, for it is found to be the Chern number associated with the Fermi projection (when the latter lies in a gap of the spectrum of the discrete Hamiltonian). We display the equivalence with the formula found for rational flux in the physics literature by using an explicit representation of the algebra of observables.The Hall effect is often modelled by considering electrons moving on a plane under the influence of a perpendicular magnetic field and a periodic potential. We show in § 2 that for both this model and the discrete model mentioned above the