Matthew describes the beginning of Jesus's ministry with the summary words, "μετανοεῖτε (repent/turn), for the kingdom of heaven is at hand" (3:2; 4:17). Why does Matthew use this command, μετανοεῖτε, at the beginning of his ministry, and how does it relate to the rest of the Gospel? What do μετανοέω and μετάνοια mean? Scholars have stated that μετανοέω in 4:17 has critical value for understanding Matthew because the verse functions as a summary statement (or key phrase) of Jesus's public ministry and teaching. This book argues the thematic significance of μετάνοια (turning/repentance) in the Gospel of Matthew. The lexical idea of μετανοέω and μετάνοια involves a turning of mind (or heart, will, thinking) and behavior, and so in turn of one's whole being and life. This opening commandment of turning (μετανοέω), especially the concept, the essence, and the contents is fully revealed throughout the body of Matthew in various ways. Discipleship, the language of righteousness, doing the will of God, changing one's heart and mind, the Great Commission, and Matthean soteriological theme convey the essence of μετάνοια and the contents of the fruit worthy of μετάνοια(3:2, 8; 4:17). The five major teaching blocks (5-7; 10; 13; 18; 23-25) teach the theme and the content of μετάνοια.