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Context: Understanding how and why programs work-not simply whether they work-is crucial. Good theory is indispensable to advancing the science of improvement. We argue for the usefulness of ex post theorization of programs. Methods:We propose an approach, located within the broad family of theoryoriented methods, for developing ex post theories of interventional programs. We use this approach to develop an ex post theory of the Michigan Intensive Care Unit (ICU) project, which attracted international attention by successfully reducing rates of central venous catheter bloodstream infections (CVCBSIs). The procedure used to develop the ex post theory was (1) identify program leaders' initial theory of change and learning from running the program; (2) enhance this with new information in the form of theoretical contributions from social scientists; (3) synthesize prior and new information to produce an updated theory. Findings:The Michigan project achieved its effects by (1) generating isomorphic pressures for ICUs to join the program and conform to its requirements; (2) creating a densely networked community with strong horizontal links that exerted normative pressures on members; (3) reframing CVC-BSIs as a social problem and addressing it through a professional movement combining "grassroots" features with a vertically integrating program structure; (4) using several interventions that functioned in different ways to shape a culture of

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