Objectives: An adverse event (AE) is a physical harm experienced by a patient due to health care, requiring intervention. Describing and categorizing AEs is important for quality and safety assessment and identifying areas for improvement. Safety science suggests that improvement efforts should focus on preventing and mitigating harm rather than on error, which is commonplace but infrequently leads to AEs. Most taxonomies fail to describe harm experienced by patients (e.g., hypoxia, hemorrhage, anaphylaxis), focusing instead on errors, and use categorizations that are too broad to be useful (e.g., “communication error”). We set out to create a patient-centered, emergency department (ED)-specific framework for describing AEs and near misses to advance quality and safety in the acute care setting. Methods: We performed a critical review of existing taxonomies of harm, evaluating their applicability to the ED. We identified and adopted a classification framework and developed a taxonomy using an iterative process categorizing approximately 600 previously identified AEs and near misses. We reviewed this taxonomy with collaborators at four medical centers, receiving feedback and providing clarification. We then disseminated a set of representative scenarios for these safety experts to categorize independently using the taxonomy. We calculated interrater reliability and performance compared to our criterion standard. Results: Our search identified candidate taxonomies for detailed review. We selected the Adventist Health Systems AE taxonomy and modified this for use in the ED, adopting a framework of categories, subcategories, and up to three modifiers to further describe events. On testing, overall reviewer agreement with the criterion standard was 92% at the category level and 88% at the subcategory level. Three of the four raters concurred in 55 of 59 scenarios (93%) and all four concurred in 46 of 59 scenarios (78%). At the subcategory level, there was complete agreement in 40 of 59 (68%) scenarios and majority agreement in 55 of 59 instances (93%). Performance of individual raters ranged from very good (88%, 52/59) to near perfect (98%, 58/59) at the main category level. Conclusions: We developed a taxonomy of AEs and near misses for the ED, modified from an existing framework. Testing of the tool with minimal training yielded high performance and good inter-rater reliability. This taxonomy can be adapted and modified by EDs seeking to enhance their quality and safety reviews and characterize harm occurring in their EDs for quality improvement purposes.