Simulation models for the interaction between weeds and crops generally are simple, empirical equations which lack generality across locations or species or are so complex and difficult to apply that their use by independent researchers is not feasible. The objective of this article is to describe and demonstrate a user-oriented model for weed-crop competition with enough detail to be general across locations and species but not so complex that independent users cannot apply it to their situations. The ALMANAC model described in this article contains the detailed functions for water balance, nutrient cycling, and plant growth as in the EPIC model, and additional detail for light competition, population density effects, and vapor pressure deficit effects which enable it to simulate the growth and seed yield of two competing plant species in a wide range of environments. It reasonably simulates the impact of infestations of johnsongrass, foxtail species, or cocklebur on yields of maize, soybean, and wheat. This model should be a useful tool for simulating management strategies related to weed control.