The first two decades of the twenty-first century witnessed the blossoming of frameworks to conceptualize "21st-century skills" both in school and career. However, there is still a lack of consensus about what we are talking about when using the term "21st-century skills," as the existing frameworks on "21st-century skills" are different from each other. Drawing upon 471 existing sources published in 2000-2017 regarding "21st-century skills," identified from six research databases, this study was intended to answer the following research question via a systematic review: What skills had been considered as "21st-century skills" in the existing educational research literature? Seven categories or domains of knowledge and skills emerged when investigating this literature. Implications and future research directions about how adaptive learning and adaptive testing in the new century can empower educational and vocational guidance were briefly discussed at the end of this paper.