Non-Functional requirements are those requirements that are not readily captured in use-cases as use-cases are typically functional (behavioural)in nature.

Also known as 'Supplementary Specifications' they are the URPS+ of the FURPS+ model (Functional, Usability, Reliability, Performance, Supportability), described by Robert Grady (Practical Software Metrics for Project Management and Process Improvement. Prentice-Hall.). The + being requirements concerning design constraints, implementation requirements, interface requirements and physical environment requirements or constraints.

Whereas 'Functional' requirements describe the actions that the system must be able to perform, 'Non-Functional' requirements describe attributes of the system or attributes of the system environment.

Many times there is confusion about some requirements whether they are Functional or Non-Functional, especially frequently we see that re-usable functionality often ends up in the supplementary specifications rather than in some kind of use case that should be 'included' in many others, as a short cut that usually leads to confusion.

We can use this Forum to discuss what constitutes a Non-Functional requirement and how and where to capture it (in formal or informal or Agile or ....... methodlogies)