PROJECT SUMMARY/ABSTRACT Shared decision making (SDM) has the potential to improve quality of care and reduce health disparities. To engage in SDM, patients must have both (1) knowledge of the treatment options, and (2) power ? the self- perceived need and capacity ? to influence decision making. SDM interventions, e.g. decision aids, increase knowledge. However, barriers to patient empowerment hinder engagement, including patients? perceptions that their personal input is not valued and doctor-patient power imbalances. Importantly, socioeconomically disadvantaged patients disproportionately experience these barriers. Understanding how decision aids address barriers to disadvantaged patients? engagement in decision making and identifying persistent barriers which can be targeted by adjunct interventions are critical steps towards reducing health disparities. In this study, we use breast cancer surgery as a model to identify and characterize barriers to socioeconomically disadvantaged patients? engagement in SDM. In order for a decision aid to be effective, patients must be able to access and review it; it is therefore critical to consider barriers to access. Yet even after successful review of a decision aid, persistent barriers may limit patients? engagement. The objective of this study is to test the effectiveness of a decision aid in increasing patient engagement in SDM and identify barriers to engagement not mitigated by the decision aid that could be targets for adjunct SDM interventions. We propose a multi-site cluster randomized trial using a stepped wedge design to enroll clinics serving a high proportion of socioeconomically disadvantaged patients within an established national community cancer research network. We will deliver a web-based decision aid via email directly to patients prior to their surgical consult. We will use mixed methods to accomplish the following specific aims: Aim 1, test the effectiveness of a breast cancer surgery decision aid in increasing patient engagement in decision making (measured by power and knowledge) in clinics serving a high proportion of socioeconomically disadvantaged patients; Aim 2, test the extent to which the effect of a decision aid on patient engagement is mediated through the mitigation of barriers, and determine if persistent barriers are disproportionately experienced by socioeconomically disadvantaged patients; and Aim 3, characterize how persistent barriers influence patient engagement in decision making in order to identify targets for adjunct interventions that could implemented in clinics serving a high proportion of socioeconomically disadvantaged patients. By understanding barriers to engagement in SDM that persist despite receipt of a decision aid, we will identify targets for adjunct interventions. Combining the routine pre-consultation delivery of a decision aid with the tailored delivery of adjunct interventions addressing persistent barriers to engagement is a sustainable model of SDM that maximizes clinics? finite resources. If proven effective, this approach will have far-reaching implications across a variety of healthcare decisions.