Highway Congestion Monitoring Program (HICOMP) in the California Department of Transportation (Caltrans) is a legislative mandated program. On an annual basis, Caltrans provides highway congestion information to the legislature for highway improvement purposes. In this paper, the state-of-the-practice and the state-of-the-art of highway congestion monitoring in California will be presented. These may include the congestion definition, congestion data collection and estimation methodologies, and congestion reporting formats. Both floating car- and loop detector-based methods of freeway congestion data collection will be discussed in detail, with an explicit evaluation of their accuracies and compatibilities as a special feature. The trend for improvement of the current practices using various emerging road - based or vehiclebased detection technologies will be finally presented as the state-of-the-art.