This paper is intended to make a more comprehensive exploration and introduction of the important concept of existence in Aquinas's philosophy and segmented into five parts: first, a generalization of “existence”; second, a glimpse of the historical background precedent to Aquinas's time, including Greek philosophy and medieval philosophy; third, the meaning and importance of existence; fourth, Aquinas's analysis of existence in his Commentaries on Aristotle; and fifth, Aquinas's theory of personality. Due to the limits of volume, here we can only address the first three parts. From these headings, it's easily seen and should be noted that the references in the paper are mostly from Commentary on Aristotle's Metaphysics.