Non-functional requirements (NFRs) are a set of specifications that describe how well a system should perform and its limitations. They are often thought of as the "-itys" and are contrasted with functional requirements, which define what a system is supposed to do. NFRs specify criteria that can be used to judge the operation of a system, rather than specific behaviors. They are architecturally significant requirements that describe the systems various quality attributes, such as speed, security, reliability, data integrity, and more.

NFRs are critical or even more critical than functional requirements that pass through the backlog. They ensure that a system meets user or stakeholder expectations, or the needs of the business. While a system can still work if NFRs are not met, it may not perform as expected. For example, each page must load within 2 seconds, or the process must finish within 3 hours so data is available by 8 a.m. local time after an overnight update.

NFRs can be categorized into different types, such as performance and scalability, security, usability, compatibility, and more. They must be quantified for clarity to ensure everyone clearly understands the goal. NFRs can be included while planning by conducting interviews, surveys, and workshops with stakeholders, using checklists, and more.