A number of studies based on the Resource Based View (RBV) consider resources as the only sources of gaining a source of a firm's sustainable competitive advantage. According to the RBV approach, there are qualities that resources must possess in order for them to realize sustainable competitive advantage for a firm. The resources must be valuable, rare, inimitable and immobile across firms. Since resources are more often common than rare, more homogenous than heterogeneous and more mobile than immobile, then firms have to combine the resources in order to develop rare and difficult to imitate processes that will act as a source of sustainable competitive advantage. In an industry where resources are common and mobile, a firm therefore needs to build competencies in order to convert these common and mobile resources into processes that are rare and immobile to create a source of sustainable competitive advantage for the firm. A problem therefore exists for firms in a homogenous industry where resources are shared and are neither rare nor heterogeneous across firms in the industry to develop sources of sustainable competitive advantage. In order for these firms to develop sources of competitive advantage with the resources available to them, they would need to develop core competencies to turn the non rare homogenous resources into rare and heterogeneous processes that competitors cannot imitate. The development of these competencies is the product of organizational cultures and values formed over time which can be explained by institutional theory. Further, a firm may not own the resources they need to form a source of sustainable competitive advantage. These resources may be owned by other firms not controlled by the firm in need of these resources which is a premise of the institutional theory. While this is acknowledged from the existing literature, there is also lack of an integrated theoretical model to demonstrate how diverse theories explaining firm strategic behaviour may be utilized to enable firms build sustainable competitive advantage. This paper proposes an integrated theoretical model for linking firm resources with core competencies and sustainable competitive advantage while providing for the role of the firm's external environment. The proposed model integrates the postulates of the RBV, RDT, Institutional Theory and Porter's five forces Model. The emerging theoretical propositions and implications for future research are discussed.