This study presents data on marital sex based on the 1988 National Survey of Families and Households. With this representative sample of United States adults (n = 7,463), we show how the incidence and frequency of marital sex change over the life course. Consistent with previous research, this study shows a decline in marital sexual incidence and frequency. Several factors contribute to this decline, including biological aging, diminished health, and habituation to sex. In multivariate analyses, age was the single factor most highly associated with marital sexual frequency. Marital happiness was the second most important predictor. Some factors found to be related to sexual frequency are associated with life changes that reduce or increase the opportunity to have sex, including pregnancy, the presence of small children, and sterilization. Controlling for age and many other factors, we found that cohabitors, married individuals who had cohabited before marriage, and those who were in their second or later marriage had more frequent sex than their counterparts who had not experienced these events. The effect of missing responses on the validity of aggregate information on sexual frequency is considered.