While 21st century skills are getting increasing traction in both academia and the professional world, a big challenge today is that there has been no consensus on the unified conceptual framework to define and measure 21st century skills. This consensus is indispensable for understanding, teaching and assessing these skills. Additionally, few frameworks have detailed the standards or benchmarks for the assessment and evaluation of the learning process and the learning outcomes with regard to 21st century skills. This paper argues with supporting evidence that instituting a taxonomy is indispensable to meeting the urging needs of 21st century skill education. Also, it brings up a unique taxonomy, the Taxonomy for Assessing 21st Century Skills (TACS), which is a framework built on all the existing frameworks explicitly on 21st century skills thus far as identified via a systematic review. It is intended to cover the minimum conceptual components, including the minimum educational objectives and benchmarks that must be considered for a dialogue or action undertaken on 21st century skill education.