In recent years, high-achieving students have received increased attention by researchers, policymakers and practitioners. However, the question of what exactly constitutes high academic achievement is not yet agreed upon by the research community. This paper provides a systematic review of how researchers studying high-achieving students since 2000 have operationalized high academic achievement in their research. In particular, we examined which performance indicators were used, whether achievement was conceived of as subject-specific or general, and which cut-off values and comparison standards were applied. The systematic database search yielded N = 309 articles, n = 55 of which were finally included in the analysis. The present study observed a diversity in the operationalization of performance. The most commonly used indicators of performance were grades and test scores, with cross-domain and subject-specific definitions both being common. Some of the studies' cut-off values were difficult to compare, but in instances where a population norm could be derived, the median proportion of high achievers was 10 percent. The study discusses that constraints on generalizability and comparability between different studies on high achievers can arise due to methodological differences. This paper concludes with recommendations for the operationalization of high achievement.