Constant elasticity of substitution (CES) is a common specification of many production functions and utility functions in neoclassical economics. CES holds that the ability to substitute one input factor with another (for example labour with capital) to maintain the same level of production stays constant over different production levels. For utility functions, CES means the consumer has constant preferences of how they would like to substitute different goods (for example labour with consumption) while keeping the same level of utility, for all levels of utility. What this means is that both producers and consumers have similar input structures and preferences no matter the level of output or utility.

The vital economic element of the measure is that it provided the producer a clear picture of how to move between different modes or types of production, for example between modes of production relying on more labour. Several economists have featured in the topic and have contributed in the final finding of the constant. They include Tom McKenzie, John Hicks and Joan Robinson.

Specifically, it arises in a particular type of aggregator function which combines two or more types of consumption goods, or two or more types of production inputs into an aggregate quantity. This aggregator function exhibits constant elasticity of substitution.

Despite having several factors of production in substitutability, the most common are the forms of elasticity of substitution. On the contrary of restricting direct empirical evaluation, the constant Elasticity of Substitution are simple to use and hence are widely used. McFadden states that;

The constant E.S assumption is a restriction on the form of production possibilities, and one can characterize the class of production functions which have this property. This has been done by Arrow-Chenery-Minhas-Solow for the two-factor production case.

The CES production function is a neoclassical production function that displays constant elasticity of substitution. In other words, the production technology has a constant percentage change in factor (e.g. labour and capital) proportions due to a percentage change in marginal rate of technical substitution. The two factor (capital, labor) CES production function introduced by Solow, and later made popular by Arrow, Chenery, Minhas, and Solow is:

where

Calculating the Rate of Technological Substitution between K and L: