The past decade has witnessed groundbreaking developments in metalloenzyme-catalyzed free radical transformations, which were previously unknown or uncommon in native metalloenzymology. Guided by mechanistic understandings from organic, organometallic, and biochemistry, an array of radical reactions has been developed using various metalloprotein catalysts based on first-row transition metal cofactors including Fe, Co, and Cu. The structural and functional diversity and the readily tunable activ