Close to one million red blood cell (RBC) units are transfused annually in Canadian hospitals, with surgical inpatients accounting for up to 44% of transfusions. There is evidence of significant variation in transfusion practice in the operating room (i.e., intraoperative). Although variation is expected based on disease severity and patient preference, inappropriate clinical care due to either under- or over-transfusion likely also contributes to significant variation. Indeed, estimates of unwarranted intraoperative RBC transfusions in the literature range from 19% to 49%, owing partly to a lack of evidence-based consensus on RBC transfusion practice in the OR. Our two systematic reviews have highlighted this gap, demonstrating a lack of evidence from trials or actionable clinical practice guidelines to inform decisions in the OR. Perhaps more importantly, avoidance of blood product exposure is an important patient-prioritized outcome that has yet to be studied empirically in the OR. As such, the observed variation in transfusion practice suggests that transfusion decision-making during surgery represents a clear and important knowledge and evidence gap. Transfusion decision-making in the OR is a complex and dynamic process that we cannot begin to improve without first understanding it. It is influenced by 1) physiologic parameters such as acute blood loss, the effects of general anesthesia, and surgical manipulation. Decision-making is also likely heavily influenced by 2) behavioural factors in the OR (heuristics, team dynamics, institutional culture), for which very little empirical work has been conducted. Finally, the importance of 3) patient input in influencing transfusion decisions is inadequately studied, given the documented disconnect between patient priorities and outcomes used in the medical literature and by clinicians. In this context, the aim of my thesis was to develop an empirical understanding of transfusion decision-making in the OR based on stakeholder perceptions and priorities, informed by an integrated patient engagement process. With this work, I address an important knowledge gap in intraoperative blood transfusion, thereby contributing to efforts to reduce variation in blood transfusion practice in surgery. It is my hope that this work will be influential in informing actionable perioperative tools to optimize blood management including providing both evidence and knowledge gaps for future research.