This randomized clinical trial is designed as an equivalence trial, i.e., the overall hypothesis tested is that the clinical performance of OptiBond XRT, as measured by the USPHS criteria described, will be equivalent to that of OptiBond FL, which has excellent documented clinical performance. In order to minimize possible participant and operator effects on the results of the study, the assignment of participants to operators and materials to teeth was randomized (see section Distribution of Restorations per Participant above). Participants were assigned to operators in order of screening.