In comparison to legacy engine technology, natural gas vehicles have become cleaner and more efficient. Improved fueling infrastructure has supported the growth of natural gas vehicles in the heavy-duty sector. The heavy-duty transportation industry greatly favors the use of diesel engine technology compared to alternative fuel strategies. Local regulations and economic incentives, however, have helped to spur adoption of natural gas vehicles in certain heavy-duty vocations. Studies have shown lower distance-specific oxides of nitrogen (NOx) emissions from the stoichiometric three-way catalyst (TWC) equipped natural gas engines compared to diesel engines equipped with diesel particulate filters (DPF) and selective catalytic reduction (SCR). This review details the progress in natural gas engine technology, presents changes to emissions rate due to technology advancements, and compares natural gas engine emissions to those of modern diesel engines.