In 1971 the first author proposed the simple ATDL urban dispersion model, which found wide use in screening applications for urban area sources. More recently, the authors have written a book, soon to be published by the AIChE, entitled "Wind flow and vapor cloud dispersion at industrial and urban sites". A chapter of that book includes the formulation of an updated simple urban dispersion model, which is the subject of the current paper. The core of the model is a Gaussian Plume and Puff Model, with the wind speed, the turbulent dispersion coefficients, and the Lagrangian turbulent time scales determined by urban boundary layer formulations based on recent measurements and models. The model has a smooth transition among three regimes: regime (1), which applies to the plume released below the urban canopy at distances while the plume dimensions are still less than the spacing between the buildings; regime (2), which applies to the plume while its lateral spread is much larger than the building spacing but most of the plume is still located below the urban canopy, and regime (3), which applies to the plume after it grows such that its vertical extent is much larger than the height of the urban canopy. The new model's predictions are compared with observations from urban field and laboratory studies and with the predictions of alternate models.