Requirements engineering (RE) refers to the process of defining, documenting and maintaining requirements to the sub-fields of systems engineering and software engineering concerned with this process.
The first use of the term requirements engineering was probably in 1979 in a TRW technical report but did not come into general use until the 1990s with the publication of an IEEE Computer Society tutorial and the establishment of a conference series on requirements engineering that has evolved into the current International Requirements Engineering Conference.
In the waterfall model, requirements engineering is presented as the first phase of the development process. Later software development methods, including the Rational Unified Process (RUP), assume that requirements engineering continues through the lifetime of a system.
Requirement management, which is a sub-function of Systems Engineering practices, is also indexed in the INCOSE (International Council on Systems Engineering) manuals.
Requirements engineering activities
The activities involved in requirements engineering vary widely, depending on the type of system being developed and the specific practices of the organization(s) involved. These may include:
- Requirements inception or requirements elicitation - Developers and stakeholders meet, the latter are inquired concerning their needs and wants regarding the software product.
- Requirements analysis and negotiation - Requirements are identified (including new ones if the development is iterative) and conflicts with stakeholders are solved. Both written and graphical tools (the latter commonly used in the design phase but some find them helpful at this stage, too) are successfully used as aids. Examples of written analysis tools: use cases and user stories. Examples of graphical tools: UML and LML.
- System modeling - Some Engineering fields (or specific situations) require the product to be completely designed and modeled before its construction or fabrication starts and, therefore, the design phase must be performed in advance. For instance, blueprints for a building must be elaborated before any contract can be approved and signed. Many fields might derive models of the system with the Lifecycle Modeling Language, whereas others, might use UML. Note: In many fields, such as Software Engineering, most modeling activities are classified as design activities and not as requirement engineering activities.
- Requirements specification - Requirements are documented in a formal artifact called Requirements Specification (RS). Nevertheless, it will become official only after validation. A RS can contain both written and graphical (models) information if necessary. Example: Software requirements specification (SRS).
- Requirements validation - Checking that the documented requirements and models are consistent and meet the needs of the stakeholder. Only if the final draft passes the validation process, the RS becomes official.
- Requirements management - Managing all the activities related to the requirements since inception, supervising as the system is developed and, even until after it is put into use (e. g., changes, extentions, etc.)
These are sometimes presented as chronological stages although, in practice, there is considerable interleaving of these activities.
One limited study in Germany presented possible problems in implementing requirements engineering and asked respondents whether they agreed that they were actual problems. The results were not presented as being generalizable but suggested that the principal perceived problems were incomplete requirements, moving targets, and time boxing, with lesser problems being communications flaws, lack of traceability, terminological problems, and unclear responsibilities.
There is no evidence that requirements engineering contributes to the success of software projects or systems. Problem structuring, a key aspect of requirements engineering, decreases design performance.  Some research suggests that software requirements are often an illusion misrepresenting design decisions as requirements in situations where no real requirements are evident.
- Alan M. Davis's extensive bibliography of requirements engineering.
- Requirements Engineering Specialist Group (RESG)
- International Requirements Engineering Board (IREB)
- IEEE 12207 "Systems and software engineering – Software life cycle processes"
- TOGAF (Chapter 17)
- Concept of operations (ConOps)
- Operations management
- Software requirements
- Software requirements specification
- Software Engineering Body of Knowledge (SWEBOK)
- Design specification
- Specification (technical standard)
- Formal specification
- Nuseibeh, B.; Easterbrook, S. (2000). Requirements engineering: a roadmap (PDF). ICSE'00. Proceedings of the conference on the future of Software engineering. pp. 35–46. CiteSeerX . ISBN 1-58113-253-0. doi:10.1145/336512.336523.
- Kotonya, Gerald; Sommerville, Ian (September 1998). Requirements Engineering: Processes and Techniques. John Wiley & Sons. ISBN 0-471-97208-8.
- Chemuturi, M. (2013). Requirements Engineering and Management for Software Development Projects. ISBN 978-1-4614-5376-5. doi:10.1007/978-1-4614-5377-2.
- Software Requirements Engineering Methodology (Development) Alford, M. W. and Lawson,J. T. TRW Defense and Space Systems Group. 1979.
- Thayer, Richard H.; Dorfman, Merlin, eds. (March 1997). Software Requirements Engineering (2nd ed.). IEEE Computer Society Press. ISBN 0-8186-7738-4.
- Royce, W. W. (1970). Managing the Development of Large Software Systems: Concepts and Techniques (PDF). ICSE'87. Proceedings of the 9th international conference on Software Engineering. pp. 1–9.
- Sommerville, Ian (2009). Software Engineering (9th ed.). Addison-Wesley. ISBN 978-0-13-703515-1.
- Méndez Fernández, Daniel; Wagner, Stefan (2015). "Naming the pain in requirements engineering: A design for a global family of surveys and first results from Germany". Information and Software Technology. 57: 616–643. doi:10.1016/j.infsof.2014.05.008.
- Ralph, Paul; Mohanani, Rahul (May 2015). "Is Requirements Engineering Inherently Counterproductive?". IEEE.
- Ralph, P. (September 2013). "The illusion of requirements in software development". Requirements Engineering. 18 (3): 293–296. doi:10.1007/s00766-012-0161-4.
- 29148-2011 - Systems and software engineering — Life cycle processes — Requirements engineering. 2011. pp. 1–94. ISBN 978-0-7381-6591-2. doi:10.1109/IEEESTD.2011.6146379.("This standard replaces IEEE 830-1998, IEEE 1233-1998, IEEE 1362-1998 - http://standards.ieee.org/findstds/standard/29148-2011.html")
- Systems Engineering Body of Knowledge