Evidence-based insight on the effectiveness of reminders in web-based data collection in online panels is scarce. Thirty-eight studies were conducted in three different online panels to examine the effect of reminders on response rates, retention rates, and two facets of response quality (i.e., item omissions and response nondifferentiation). The studies were summarized using a meta-analytical approach including moderator tests (N = 246,040). Results indicated that reminders in web-based data collection increase response (i.e., from 49.5% to 65.6%, ES pd = .152). However, reminders somewhat reduce retention (i.e., from 98.0% among prereminder respondents to 97.2% among reminded respondents, ES pd = -.008), which might be explained by differences in motivation and computer literacy between unreminded and reminded respondents. No differences were found between prereminder respondents and reminded respondents in item omissions and response nondifferentiation.