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Customer requirements are the foundation upon which a software system is built. These requirements, derived from the customers’ needs and desires, are used to both guide the development of the system and to determine if the completed system is what the customer requested. Because of its importance, requirement specification has become a research area known as Requirements Engineering (RE) both in Software Engineering [Zave, 1997; Nuseibeh & Easterbrook, 2000] and Systems Engineering [Dorfman, 1990; Chandrasekaran & Kaindl, 1996].

The primary goal of requirements engineering is to capture and represent system requirements so that they can be traced through to both implementation and testing to ensure that the resulting system does what the customer has requested. Requirements are commonly broken into two types: functional requirements (FRs) that correspond to desired functional capabilities of the system and non-functional requirements (NFRs) that describe desirable overall properties that the system must have (such as being “costeffective” or “user-friendly”). NFRs generally are not directly related to specific system components and often involve aggregate system behavior [Manola, 1999].