Passive NOx adsorbers have been proposed to help address cold start NOx emissions. Although the original concept was proposed earlier, it is not until the last few years that more significant efforts focused on understanding the fundamentals of this technology. Several groups of potential PNAs have been evaluated for low temperature NOx storage and subsequent release, and the laboratory-scale experimental testing results appears to be promising. This review will summarize the currently reported materials for PNA application, their unique behaviors when operating under different conditions, systematically evaluate and compare their performances and discuss the relevant reaction chemistry. The discussion around each specific topic is based on literature reported experimental data from testing and characterizing potential PNA candidates and will put more emphasis on the recent advances in the fundamental understanding of this technology. Since at this stage the mechanism of the adsorption and desorption processes is not fully understood, performance evaluation and reaction chemistry will be discussed independently. Unresolved issues are listed at the end of each section as an attempt to summarize the critical unknowns within a specific research topic.