There is no direct scale for measuring software functionality. One must instead decide which aspects of software to measure, that seem to capture functionality; how to measure those aspects; and how to combine the measurements into an overall measure of application size.

Many methods for how to do this have been proposed, beginning with Allan Albrecht's invention of Function Point Analysis in 1979. The methods vary in what things are measured and how the measuring is done; and also in usefulness, applicability, and industry acceptance. Two methods have achieved significant industry acceptance, and a third is on its way.

In this chapter we trace the evolution of functional size measurement, from Albrecht's original ideas through to the recent development of ISO standards. We describe Albrecht's method, and what has been learned about it through extensive experience and empirical research. We present some interesting variants on Albrecht's method that did not achieve widespread success, as well as the two other methods that can now be described as mainstream. We describe research concerning function points, and conclude with thoughts on the current status of functional size measurement and some expectations for future work.