The primary objective of this literature review is to provide a glimpse at technology changes for diesel and natural gas (NG) engines in the HDV space that are expected over the next 10 to 15 years as India transitions to world-class criteria pollutant emissions standards and improved efficiency becomes a larger focus as a result of fuel efficiency regulation. The paper begins by summarizing the engine technology pathways associated with transitioning from the current pollutant emissions standard (Bharat Stage IV in select cities, Bharat Stage III nationwide) to Bharat Stage VI, which is expected to be nearly identical to the Euro VI standard. It then reviews efficiency improvements currently being deployed plus others in the pipeline for the post-2020 timeframe in developed markets. The particular focus is the United States and Canada, where fuel efficiency regulations—specifically, engine-specific performance standards—are driving fuel-saving technology advancements in a number of areas on engines and aftertreatment systems