PROJECT SUMMARY/ABSTRACT Pulse oximeters are used in nearly every pediatric hospitalization to non-invasively monitor arterial blood oxygen saturation. Pulse oximetry data are essential to clinical decision-making, informing decisions from the use of supplemental oxygen to hospital admission to transfer to intensive care. Research indicates that pulse oximeters may overestimate arterial oxygen saturation in hypoxemic patients with dark skin colors more than in those with light skin colors. Prior studies have been limited by availability of data and use subjective skin color measurement methods. In some studies, self-identified Black race has been associated with diminished pulse oximeter accuracy, without measuring skin color at all. The majority of existing studies were conducted in adults; however, characteristics of children's skin, such as skin thickness and melanin concentration, may contribute to different results in children. The overall objective of this study is to develop a comprehensive understanding of the relationship between skin color and pulse oximeter accuracy in children in the clinical setting. The results may inform recommendations for clinicians as well as policies and action from industry and the Food and Drug Administration. This multisite, prospective, observational study will be conducted in a population of children under 18 years old who are undergoing cardiac catheterization. Children undergoing cardiac catheterization are a unique population in which to study pulse oximeter accuracy because they have a wide range of baseline arterial oxygen saturations secondary to their cardiac conditions, allowing for evaluation of pulse oximeter accuracy across a range of arterial oxygen saturations. Children undergoing cardiac catheterization are continuously monitored with pulse oximeters and also have direct measurement of arterial blood oxygen as part of routine care. Data will be collected directly from participants and extracted from their medical record. Skin color will be objectively measured using colorimetry. The specific aims of the study are twofold; first, to quantify pulse oximeter accuracy in children with varying skin colors and second, to explore the mechanistic link between self-identified race and pulse oximeter accuracy. The second aim will assess whether skin color mediates the relationship between race and pulse oximeter accuracy that was demonstrated in earlier studies. The results of this study will contribute much needed knowledge about pulse oximeter accuracy in children of varying skin colors to on-going efforts to reduce disparities and inequities in healthcare.