In this text we have developed the basic framework of input–output analysis and many of the extensions and applications that followed Professor Leontief’s seminal work defining the field nearly seven decades ago (chronicled in Appendix C). To this day, input–output and its extensions endure by themselves as tools for many kinds of economic analysis. The basic framework often comprises a fundamental component of many other types of economic analysis as well, such as econometric general equilibrium and planning models, and extensions also include applications to broader social accounting problems, such as social accounting matrices, ecological analysis, tracing material or energy use and flow throughout an economy measured in physical terms, or defining material or environmental footprints for firms, regions, or nations. We have captured many, and perhaps most, but not all these extensions and applications to varying degrees of detail in this text.