High-emitting vehicles (HEVs), also known as high emitters, are responsible for a large proportion of total mobile-source emissions, even though they represent a small portion of the total vehicle fleet. Studies conducted in the 1990s indicated that between 7% and 12% of the vehicle fleet accounted for 41% to 63% of total carbon monoxide (CO) emissions; 10% of vehicles were responsible for 47% to 65% of hydrocarbon (HC) emissions; and 10% of vehicles were responsible for 32% of nitrogen oxide (NOx) emissions. However, these conclusions were confined both temporally and spatially because the studies used remote-sensing data or inspection and maintenance test data that were collected at a specific location and time. Consequently, the objective of this study is to quantify the contribution of HEVs at a network level by using comprehensive modal emission models (CMEMs) and Virginia Tech-Micro (VT-Micro) microscopic vehicle emission models. This study applies two approaches besides the use of the CMEM and VT-Micro models. First, the study quantifies the HEVs' contribution by using different predefined drive cycles. Second, a simulation of a highway and arterial network is applied. The study results demonstrate that HEV emission contributions to total HC and CO emissions are consistent with what is reported in the literature. However, the contribution of NOx emissions is greater than what is reported in the literature. The study demonstrates that the contribution of HEVs to total vehicle emissions is dependent on the type of roadway facility (arterials versus highways), the background normal vehicle composition, and the composition of HEVs.