To design gas extraction and utilization facilities and to calculate cost and revenues, the gas production with time has to be predicted as accurate as possible. This is also of great importance for the calculation of the greenhouse gases. Gas production curves are in general predicted using landfill gas models. There are different kinds of models described in the literature that are presented in this chapter. Mostly two models as the Landfill Gas Emissions Model and first-order decay Intergovernmental Pannel of Climate Change model are used. As always, when using models, the availability of as accurate as possible input data is essential. There are input data, presented in this chapter, that might be used, but they always should be critically validated, because they might not always be appropriate for each case.