The only thing constant is change. This is certainly more true of software systems than almost any phenomenon. Not all software systems change in the same way or at the same rate. Some constantly undergo major modifications and others remain untouched for years at a time. Identification and understanding of these differences in dynamic software system behavior (i.e., software evolution) can improve software engineering and systems management. Measurement is key to understanding any phenomenon. Software volatility, a characteristic of software behavior, describes the changeable nature of software. By rigorously defining, evaluating, and validating a measure of software volatility, we can expand our understanding of the evolutionary processes constantly transforming software systems.