Agency theory, in its mainstream version, is one of the dominant theories underpinning much of the current understanding of corporate law and corporate governance. In the dominant, but rather limited and overly shareholder-focused variant, agency theory lends support to the social norm of shareholder primacy: a main barrier to internalizing environmental externalities. Insights into the nature of the shareholders and the rise of ‘secret’ agents and principals, give reason to question the legitimacy of the rise of shareholders as corporate decision-makers. Further questions are raised when what from a real-life point of view occurs when one enterprise or business entity is split up into various legal entities, as is often the case in practice. Further fragmentation of accountability takes place through use of networks and supply chains. This chapter advances a broader, entity-based view and a redefined agency theory, offering a meaningful contribution to the necessary discussion on how to internalize environmental externalities in any product-based economic activity. This would involve a life-cycle based mapping of the products as an intrinsic element of drawing up the boundaries of the entity within which the agency issues of relevance to environmental, social and financial corporate sustainability should be identified and analysed.