In order to adequately monitor neutralization breadth and potency and to compare and prioritize immunogens, assays are needed that are sensitive, quantitative, high throughput, and have correlative value. Substantial improvements have been made in the past several years in assay technology and in available reference reagents. Thus, cumbersome and expensive assays using peripheral blood mononuclear cells (PBMC) and uncloned viruses are being replaced with assays that utilize molecularly cloned Env-pseudotyped viruses and genetically engineered target cells lines [2, [42] [43] [44] [45] . This new technology affords greater sensitivity, reproducibility, high throughput, and cost-effectiveness compared to PBMC assays, and as a result, it has been responsible for an explosion of new data. Steps are being taken by the Collaboration for AIDS Vaccine Discovery to transfer this new technology to multiple laboratories around the world and to implement a validated proficiency testing program to assure inter-laboratory equivalency in assay performance.