PROJECT SUMMARY/ABSTRACT The objective of this multicenter study is to develop and validate a novel, non-invasive biomarker for pediatric NAFLD. The specific aims are to recruit a cohort of 80 youth (6-21 years of age) with NAFLD and 80 controls with obesity and study their skin surface lipids using state of the art lipidomic analyses to: 1. Determine the cutaneous lipids that differentiate youth with NAFLD from controls; 2. Investigate the skin lipidome that detects the presence of advanced disease (non-alcoholic steatohepatitis [NASH], fibrosis) among those with NAFLD; and, 3. Further support the use of skin surface lipids as a biomarker of NAFLD in children by studying its repeatability and variability. Participants will undergo magnetic resonance imaging with proton density fat fraction (MRI-PDFF) at baseline to determine the presence/absence of NAFLD. This methodology has been shown to be accurate in determining the presence of hepatic steatosis. They will then have their cutaneous lipidome sampled using tape stripping. The latter is a rapid (<5 min), painless procedure that samples the lipids of the most superficial layer of the skin (from the stratum corneum and the lipids secreted by the sebaceous glands). All patients with NAFLD will have had prior histologic confirmation of their disease, which will be re-reviewed and scored by a single pathologist, with expertise in pediatric NAFLD, who will be masked to patient clinical and demographic characteristics. Untargeted lipidomic analyses of the collected skin samples will be performed using ultra high-performance liquid chromatography-quadrupole time of flight mass spectrometry to determine the skin surface lipids that alone or in combination predict the presence of NAFLD. The performance of the cutaneous lipidome in predicting the presence of NAFLD in youth with obesity will be compared against that of the currently used screening test, namely serum alanine aminotransferase levels. Among those with NAFLD, the performance of skin surface lipids in predicting the presence of NASH and fibrosis will be assessed. Lastly, a subset of patients with NAFLD will have repeat skin sampling: a. on the same day from the opposite arm, to test the repeatability of cutaneous lipidome, and b. 1 month later, to test the variability of the lipidome as a biomarker. Variables that may affect the skin surface lipids, such as sex, age/pubertal status, change in diet, obesity severity and insulin resistance status, will be assessed and controlled for in these analyses. A non-invasive biomarker for pediatric NAFLD is urgently needed, due to the staggering prevalence of this disease and the fact that the currently available invasive diagnostic modalities (liver biopsy) are not practical and hinder therapeutic discovery and progression.