Recent 2010 emissions standards for heavy-duty engines have established a limit of oxides of nitrogen (NOx) emissions of 0.20 g/bhp-hr. However, it is projected that even when the entire on-road fleet of heavy-duty vehicles operating in California is compliant with 2010 emission standards, the National Ambient Air Quality Standards (NAAQS) requirement for ambient particulate matter and Ozone will not be achieved without further reduction in NOx emissions. The California Air Resources Board (ARB) funded a research program to explore the feasibility of achieving 0.02 g/bhp-hr NOx emissions. This paper details the work done to explore the feasibility of various configurations of Traditional Technology (diesel oxidation catalyst-diesel particulate filter-selective catalytic reduction (SCR)) and Advanced Technology (passive NOx adsorber or diesel oxidation catalyst – SCR on Filter – SCR) to demonstrate ultra-low NOx emissions from heavy-duty engines. Active and passive performance modifiers were also evaluated to demonstrate low NOx emissions, including heated dosing, gaseous dosing, and supplemental heat addition devices. Data will be presented showing comparison in NOX reduction capability and potential fuel penalty. All testing was conducted on the FOCAS-HGTR, which is a full flow, transient gas reactor bench for testing full sized catalyst systems.