This meta-analysis examines the effects of sorting secondary students into different tracks ("between-school" tracking) or classrooms ("within-school" tracking) on the efficiency and inequality levels of an educational system. Efficiency is related to the overall learning achievement of students, whereas inequality can refer to "inequality of achievement" (i.e., the dispersion of outcomes) or "inequality of opportunity" (i.e., the strength of the influence of family background on student achievement). The selected publications are 53 analyses performed in the period from 2000 to 2021, yielding 213 estimates on efficiency and 230 estimates on inequality. The results show that the mean effect size (Hedge's G) of tracking on efficiency is not statistically significant (G = -0.063), whereas it is significantly positive (G = 0.117) on inequality. We further set out to explain variation in effect sizes by (a) policy characteristics, (b) the operationalization of main variables, (c) the research design, (d) the set of control variables included in the statistical analyses, and (e) the quality of the study, year of publication, and publication status (peer reviewed or not peer reviewed).