Reducing the greenhouse gas (GHG) emissions from modern energy systems is critical to manage climate change. For systems such as heavy-duty or marine engines, where electrification is not yet possible, natural gas can be an attractive alternative. For example, natural gas as an engine fuel can result in lower carbon dioxide and particulate emissions than diesel fuels, and significantly lower sulfur emissions than marine fuels. Several fueling technologies exist for natural gas; however, the overall performance is very closely related to the engine operating strategy, and excessive methane emissions can lead to increased GHG emissions. In this work we aim to reduce the GHG emissions from natural gas engines using a combination of lab based engine experiments, as well as field measurements of GHG emissions from in-use engines. The lab-based studies will develop methodologies for in-use emission characterization and strategies for emissions reductions, based on operating conditions of field engines. This will provide technologies for low GHG emission transportation systems and provide quantitative emission characterization for inventory and policy development purposes.