IT system is a general term for all software based business applications used in enterprises. IT systems support the actions of an enterprise by processing information about the enterprise and its environment and by providing this information to the enterprise and its stakeholders. An enterprise's actions have a direct influence on its ability to succeed in its environment. IT systems, therefore, have a direct influence on the enterprise's long term success. Hence, IT systems are considered to be of strategic importance in most contemporary enterprises. Although enterprises, most of the time, attempt to maintain their identity, forces within them and in their environment push them to change. Enterprise strategy therefore seeks to balance the need to remain the same with the need to change. This balance is maintained by specifying change that the enterprise is capable of sustaining and that the enterprise believes are necessary for its continued success. The design of IT systems should reflect this need for stability and change. The requirements of an IT system are the description of what the IT system will be like and how it will behave. The initial understanding of the requirements is called early requirements. Early requirements define the problems the enterprise is trying to solve and sketch the possible solutions to these problems. An envisioned IT system is often part of these solutions. Enterprise Architecture (EA) and Goal-Directed Requirements Engineering (GDRE) propose methods for defining early requirements by considering the goals of the enterprise and its stakeholders. The concept of goal is used to give structure to the different perspectives on the enterprise defined by its stakeholders and to express the resulting requirements for the IT system. The EA and GDRE literature does not propose a conceptual foundation that gives meaning to the different kinds of goals specified by the EA and GDRE methods as well as how these goals are formed and modified in enterprises. Moreover, EA and GDRE methods are often influenced by Business Process Reengineering (BPR) which is known for specifying radical, often unsustainable change. The resulting early requirements specify goals that could have been changed or that specify too much change for the enterprise. The Lightswitch approach, described in this thesis, was designed as a tool for IT system designers to create initial requirements taking into account the enterprise's needs for stability and change in terms of goals. The Lightswitch approach consists of a conceptualization and a modeling framework. The Lightswitch conceptualization explains the goal-directed behavior of enterprises from the standpoint of the maintenance of success in a changing environment. It is based on General Systems Thinking (GST) and Cybernetics principles. Combined, these theoretical perspectives offer an evolutionary viewpoint describing enterprises as systems that maintain their internal order by regulating their relationships with other systems. GST and Cybernetics offer a set of principles with which to understand this regulation. These principles are used in the Lightswitch conceptualization to explain how enterprises regulate their relationships with their stakeholders in order to remain successful. The Lightswitch conceptualization provides an explanation, in an enterprise context, of the different kinds of goals specified in the EA and GDRE literature. The conceptualization forms the theoretical background of the Lightswitch modeling framework, a goal-directed modeling framework that enables IT system designers to specify early requirements for an IT system based on the enterprise's regulation of its relationships with its stakeholders. The Lightswitch framework complements existing EA and GDRE methods by enabling designers to model both the stability and the changing nature of the relationships of the enterprise with its stakeholders. These models help designers to better understand the enterprise's goals, to propose changes to these goals if deemed necessary, and to specify early requirements for the enterprise's IT systems in the form of high-level goals. This thesis contributes an original conceptualization and method to EA and GDRE. Concretely, the Lightswitch approach consists in reflecting on the conditions that brought the enterprise-under-consideration to be what it is today, to analyze how well it is adapted to its present conditions, and to attempt to foresee some of the challenges it may face in the future. The early requirements for the IT system should reflect these past, present and future perspectives. We present three case studies in which the Lightswitch approach was used to specify the early requirements for an enterprise IT system. Two of the case studies were performed in industrial settings.