Coke formation and its effect on the catalyst is modeled at three levels: the active site, the particle, and the reactor. At the site level, information on the chemical pathways leading to the deactivating agent and on its interaction with the sites is essential. At the particle level, the presence of concentration profiles and the influence of the structure of the pore network have to be accounted for. The latter aspect is dealt with through percolation theory and the effective medium approximation. At the reactor level, the non-steady-state behavior caused by the deactivation of the catalyst is illustrated. The parallelism between the modeling of deactivation by coke formation and a specific type of poisoning leading to pore blockage is briefly discussed.