In systems engineering and requirements engineering, a non-functional requirement (NFR) is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviours. They are contrasted with functional requirements that define specific behavior or functions. The plan for implementing functional requirements is detailed in the system design. The plan for implementing non-functional requirements is detailed in the system architecture, because they are usually architecturally significant requirements.

In software architecture, non-functional requirements are known as "architectural characteristics". Note that synchronous communication between software architectural components entangles them, and they must share the same architectural characteristics.

Broadly, functional requirements define what a system is supposed to do and non-functional requirements define how a system is supposed to be. Functional requirements are usually in the form of "system shall do <requirement>", an individual action or part of the system, perhaps explicitly in the sense of a mathematical function, a black box description input, output, process and control functional model or IPO model. In contrast, non-functional requirements are in the form of "system shall be <requirement>", an overall property of the system as a whole or of a particular aspect and not a specific function. The system's overall properties commonly mark the difference between whether the development project has succeeded or failed.

Non-functional requirements are often called the "quality attributes" of a system. The emergent properties of a system are classified as non-functional requirements. Other terms for non-functional requirements are "qualities", "quality goals", "quality of service requirements", "constraints", "non-behavioral requirements", or "technical requirements". Informally these are sometimes called the "ilities", from attributes like stability and portability. Qualities—that is non-functional requirements—can be divided into two main categories:

As non-functional requirements are all requirements that do not fall into the functional requirements category, they also include both characteristics of the functions and constraints on the system such as non-design items of statutory, regulatory, standards and protocols, or other external requirements.

It is important to specify non-functional requirements in a specific and measurable way.

Common non-functional classifications, relevant for all types of systems include

Specific type of systems explicitly enumerate categories of non-functional requirements in their standards^{[to be determined]}