There is little consensus on what constitutes adequate health insurance coverage. The concept of a lack of adequate coverage, or underinsurance, is a matter of ongoing debate. A measure of adequate coverage is of critical importance as the nature of health insurance products evolves. Changes to health coverage include more direct out-of-pocket spending by consumers and a reduction of covered benefits. This article updates and extends an earlier review of underinsurance measurement published in 1993. We present a conceptual approach to measuring underinsurance and provide a review of the empirical findings obtained from the application of these approaches. A discussion of the limitations in the selection of a measurement approach includes a review of the extant data sources used. We recommend a national effort to develop a consistent approach to monitor changes in the economic and structural dimensions of health insurance coverage with a concerted effort to define and measure underinsurance.