Review Exhaust Gas After-Treatment Systems for Gasoline and Diesel Vehicles Liye Bao 1, Jihua Wang 2, Lihui Shi 2, and Haijun Chen 1, * 1 College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China 2 Valiant Co., Ltd., Yantai 264006, China * Correspondence: chenhj@nankai.edu.cn     Received: 5 October 2022 Accepted: 10 November 2022 Published: 25 December 2022   Abstract: Exhaust gases released from vehicle engines have been a major cause of air pollution, and the emission limits have become much stricter in recent years due to a worldwide concern about the impact of air pollution on public health. These regulations have been complied to minimize the emissions of carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides (NO x) and particulate matter (PM) from gasoline and diesel vehicles engines. Different after-treatment systems (ATS) have been developed for the treatment of exhaust gases from gasoline and diesel engines, respectively. The ATS for gasoline engine based on the three-way catalysts (TWCs), as well as the ATS for diesel engines including diesel oxidation catalysts (DOC), selective catalytic reduction (SCR), diesel particulate filters (DPF) and ammonia slip catalysts (ASC), are summarized in this mini-review.