Process principles, primarily material and energy balances, have been the foundation of the teaching and the practice of chemical engineering since the inception of chemical engineering as a separate profession. Before World War II, process principles were empirically applied primarily using slide rules, mechanical calculators, graphs and tables. After World War II, a brief interlude of analog computers occurred, but their complexity to program (with wires) led to a more rational approach to solving material and energy balances that paralleled the development of digital computers. Today extremely large sets of material and energy balances are solved in the industrial application of process analysis, design, and optimization. Nevertheless, process principles, by in large, continue to be taught using ‘hand solutions' of simple process equations, or their simplification for more complex processes, based on the argument that we need to avoid having students fill in dialog boxes, and instead provide students with a general understanding of material and energy balances for process systems. This presentation will focus on this evolution of the teaching of process principles from the early days based on precomputer methods (e.g., textbooks by Walker, Lewis, McAdams; Gilliland; and Hougen and Watson.) followed by the book of Henley and Rosen which was among the first to employ the more systematic approaches used today.