The increasing ubiquity of information systems in organizations has been accompanied by a rise in users adopting technology that is not officially mandated (often known as shadow IT). This study examines the emergence and consequences of such locally-driven but centrally-unintended adaptations of an organization’s collection of IT assets. This phenomenon, referred to as “IS portfolio drift, can be costly: the expenditure on developing, purchasing and supporting the officially-mandated applications is squandered, while additional funds are needed to maintain the shadow systems. Decision quality could be reduced, because of uncertainty over data provenance. However, portfolio drift can also be beneficial: it reflects employees’ innovativeness in adapting to environmental change or in using emerging technologies to enhance their performance. I use a practice theory perspective to highlight how portfolio drift is an ongoing, macro-level outcome of the micro-level actions of various agents. The process of governing an IS portfolio is influenced by the relative allocation of power in an organization, how this allocation came to be, and how it changes. Ten case studies are used to delineate a process theory of portfolio drift, and explain the situated practices that steer the portfolio-in-use to meet the goals of the agents involved.