The impact of black carbon emissions on human health and climate are of global interest. Despite this, black carbon emission factors for marine engines and fuels are uncertain because of the challenges of measuring black carbon emissions. Published black carbon emission factors can vary more than ten-fold, creating uncertainty in estimating impacts on climate and health. To address this uncertainty, this comprehensive study was undertaken by the International Council on Clean Transportation (ICCT), funded through the Climate and Clean Air Coalition and the U.S. Department of Transportation Maritime Administration (MARAD), to measure black carbon emissions from marine engines and to better understand how engine operating conditions, marine fuel choice, and vessel operations (e.g., vessel speed reduction) impact black carbon emission factors.