The Tier 2 vehicles program, defining a variety of emission standards, concepts and provisions, of the EPA in the United States has been phased in for all light-duty vehicles (LDVs), light-duty trucks (LDTs) and medium-duty passenger vehicles (MDPVs) from model year 2004, and this program will be continued until 2009, depending on the vehicle categories. Ultimately, not only should manufacturers of LDVs, LDTs and MDPVs and their importers in the United States meet the new Tier 2 standards, but all exporters outside the United States, such as Korean car makers, must also certify their vehicles by using this standard program. The principal rule for successfully applying this program to all the LDVs, LDTs and MDPVs is the use of the same Tier 2 standards if these vehicles are included in the same weight rating category, irrespective of fuel and engine types being used. This review provides an indepth discussion of key issues and provisions related to Tier 2 vehicles with engine measure strategies for automotive engineers and related academic researchers who are particularly interested in investigating how manufacturers will develop, certify, produce, and market their Tier 2 vehicles. A detailed mechanism for the phase-in of the Tier 2 standards to different vehicle weight categories will be discussed in this review, and the major difference between the US Tier 2 standards during the phase-in years and the EURO ones will be substantially compared. Great roles in meeting the future Tier 2 emission standards using numerous commercial and emerging catalytic technologies and their challenges to future Tier 2 vehicle applications will be extensively discussed.