Problems that originate in requirements are common and expensive in software projects, and they are a recurring cause of late delivery, budget overruns and cancellation. This paper is a narrative overview of requirements engineering (RE) as a means of reducing these risks. It outlines the core RE activities (elicitation, analysis and negotiation, specification, validation and change management) and the kinds of statements a requirements document contains, including the distinction between requirements, domain properties, assumptions and definitions. It reviews the evidence on the cost of requirements defects, together with the main criticisms of that evidence, and compares how the waterfall, spiral, iterative (Rational Unified Process) and agile lifecycles treat requirements. It then discusses stakeholder identification, elicitation techniques and common obstacles to elicitation, and describes practical ways of turning vague or conflicting stakeholder statements, including security and performance expectations, into precise, verifiable and prioritized requirements. The paper is intended as an introduction for practitioners and students who work with requirements but are not RE specialists.

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