This is a chapter from the free textbook "Economic Principles in Cell Biology" Resource balance analysis models are cell models based on three basic constraints formulated at genome-scale: stationary fluxes (balancing production and consumption fluxes, uptake and excretion fluxes, as well as compound dilution by cell growth); flux coupling constraints relating fluxes to the amounts of catalyzing enzymes (or other machines); and density constraints, limiting molecule amounts in cell compartments, or molecule concentrations. These constraints narrow down the solution space predicted by FBA towards more physiological solutions. Large resource allocation models build on the same principles, and have been implemented as different variations (RBA models in a narrow sense, ME-models, and pc-models).