North Dakota's input-output model has become an integral part of many economic research activities involving state issues. Since its development to analyze the economic impacts associated with irrigation development in the 1960s, the model has been updated and used to analyze the effects of a wide variety of projects in North Dakota, Because the model is used and referenced frequently by economic researchers, the nontechnical audience often requests additional information to explain the input-output model and the theory behind it. The purpose of this report is to explain the principles of input-output analysis, to describe the structure of the North Dakota model, and to explain how to interpret the results that might be found in a feasibility or economic contribution study. This report was designed to be a companion document that can be used in conjunction with any report or presentation involving the North Dakota input-output model.