We explore the correlates of noncoverage and nonresponse in an Internet survey conducted as part of the Health and Retirement Study (HRS), a panel study of persons 50 years old and older in the US. About 30% of HRS respondents indicated they used the Internet. Of these, 73% expressed willingness to do a Web survey. A subset of this group was subsequently sent a mailed invitation to participate in a Web survey and 78% completed the survey. Using multivariate models, we find significant demographic, financial, and health-related differences in access, consistent with other research. There are fewer differences in willingness (given access) and response (given willingness). However, disparities in health and socio-economic status persist after controlling for demographic differences in coverage and response. Weighting on demographics alone is thus unlikely to yield a representative sample in such surveys. Noncoverage (lack of access to the Internet) appears to be of greater concern than nonresponse (unwillingness to participate given access) for representation in Internet surveys of this age group.