Controlled radical polymerizations such as atom transfer radical polymerization (ATRP) have revolutionized and revitalized the field of synthetic polymer chemistry over the past two decades. The transition metal catalysts required for ATRP, however, represent a drawback, since they can be toxic and environmentally unfavorable. A sustainable approach to catalysis involves the use of enzymes for numerous reasons: Enzymes are derived from renewable resources, most are non-toxic, function under very mild reaction conditions and often possess high catalytic efficiency and selectivity. We have discovered that metal-containing enzymes such as horseradish peroxidase, hemoglobin and laccase are able to catalyze ATRP. Biocatalytic ATRP (bioATRP) is the first evidence that enzymes can control radical polymerizations. Moreover, this “ATRPase activity” represents a previously unknown enzymatic activity of these promiscuous proteins. Apart from deciphering fundamental aspects of ATRPase activity, the Bruns group is constantly searching for those applications in which biocatalysts can outperform conventional ATRP catalysts. For example, laccase allows the controlled radical polymerization of N-vinylimidazole, an industrially relevant monomer that cannot be polymerized by classic ATRP in a controlled way because it strongly complexes copper ions. In contrast, the copper centers of laccase are within the enzyme, so that poly(N-vinylimidazole) is synthesized with low dispersities and predetermined molecular weights. Enzymes can also be separated easily from polymer products due to their different physicochemical properties, opening new routes to metal-free polymers, which are essential for many biomedical applications and for energy materials.