A structure of logic used for synthesising and analysing processing schemes in the chemical and allied industries, in which the basic underlying concept is that all processing schemes can be composed from and decomposed into a series of individual, or unit, steps. If a step involves a chemical change, it is called a unit process; if physical change, a unit operation. These unit operations are utilised for certain definite functions wherever they are employed, and their use cuts across widely different processing applications, including industries as diverse as manufacture of chemicals, fuels, pharmaceuticals, pulp and paper, processed foods, and primary metals.

The unit operations approach thereby serves as a very powerful form of morphological analysis, which systematises process design, and greatly reduces both the number of concepts that must be taught and the number of possibilities that should be considered in synthesising a particular process.