The advent of controlled radical polymerizations has made polymer science a key discipline for the preparation of nano-, biomedical-, and high tech-materials. Atom transfer radical polymerization (ATRP) is one of the most widely applied controlled radical polymerization. However, an ongoing quest is to develop ATRP reaction conditions that allow reducing the amount of catalyst needed, or to replace the currently used transition metal complex catalysts with less toxic ones. Using enzymes as catalysts is a classic strategy in the green chemistry approach, and many enzymatic polymerizations are known. However, controlled radical polymerizations that are catalyzed by enzymes or proteins were not known until our discovery that the metalloproteins horseradish peroxidase and hemoglobin can polymerize vinyl-monomers under conditions of activators regenerated by electron transfer (ARGET) ATRP. In this book chapter, we review the emerging field of biocatalytic ATRP.