Vancomycin is a commonly administered antibiotic in critically ill children, but it can have unwanted side effects on the kidney (acute kidney injury). There are currently no effective methods to prevent development of acute kidney injury during vancomycin therapy. This study will collect blood and urine samples from children in the CHOP pediatric intensive care unit being prescribed vancomycin in an effort to describe how vancomycin behaves in the blood of critically ill children and to understand the relationship between new tests of kidney injury and function, called biomarkers, and the amount of vancomycin in the body. We hope this study will ultimately help improve the safety and efficacy of intravenous vancomycin administration in children by allowing us to develop an approach to provide personalized vancomycin dosing in critically ill children.