The Oregon Department of Transportation (ODOT) standard specifications require a minimum density for the construction of dense-graded hot mix asphalt (HMA) pavements. Currently, the standard specifications call for density measurements for quality control (QC) and quality assurance (QA) testing to be made using nuclear density gauges that are calibrated using reference blocks and correlated (adjusted) to densities from pavement cores. QC measurements (performed by the contractor) are verified by QA measurements (performed by ODOT); then ODOT utilizes the contractor's QC results for determining acceptance of the pavement (in conjunction with other specification criteria). Hence, appropriately accepting an HMA pavement, based on the in-place density criterion, relies on the accuracy of the density measurements. However, density measurement results using nuclear gauges have been observed by ODOT to be questionable on a number of projects, and repeatability and reproducibility with the same gauge and between gauges has been unattainable. The overall objective of the project described herein was to develop a system that accurately quantifies density of dense-graded HMA pavements. More specifically, the objectives were to: 1) investigate the efficacy of the various methods used by ODOT and other agencies for determining in-place HMA density; 2) assess current practices used by ODOT and other agencies for determining in-place HMA density using nuclear gauges; 3) conduct field and laboratory testing and analyses to determine the most accurate and reliable state-of-the-practice means for determining in-place HMA density; 4) provide recommendations for changes to current practices to improve accuracy and reproducibility of in-place HMA density measurements using nuclear gauges; and 5) provide recommendations for alternate means for determining in-place HMA density. To satisfy the first objective, a literature review was conducted and summarized herein. For the second objective, practices employed by ODOT were observed and assessed. For the third objective, state-of-the-practice methods used to measure HMA density (i.e., tests on core samples using the saturated surface-dry and automatic vacuum sealing methods, nuclear density gauge measurements and electromagnetic density gauge measurements) were investigated. Numerous statistical comparisons of the results were made to determine the best combination of measurement methods to ensure accurate assessment of HMA density for a variety of construction scenarios. Finally, the findings from the first three objectives were used to formulate the recommendations identified in the fourth and fifth objectives. Some of the significant findings that are based on the supporting evidence from the research are as follows: 1) Nuclear gauge densities should be adjusted to core densities for future in-place density testing of HMA pavements; 2) Cores should be tested in accordance with CoreLok testing procedure and not by using the SSD methods; 3) Cores should be extracted from the overlapping portion of the footprints of the nuclear gauge measurements; 4) It is sufficient to take two nuclear gauge readings (perpendicular and parallel to the direction of paving) rather than four readings; 5) Under certain conditions, the core adjustment factors obtained from the bottom lift can be used to adjust the nuclear gauge densities on at least the next two overlying lifts of the same pavement; and 6) The electromagnetic gauge adjustment factors could be used on more lifts than nuclear gauge adjustment factors to adjust measurements; and, electromagnetic gauge densities were not significantly different from core densities.