<p>Artificial metalloenzymes (ArMs) are proteins engineered to contain metal cofactors that often catalyze chemical reactions rarely or never observed in natural biological processes. They hold promise for applications including fine-chemical production, control of cellular function and therapeutics. Many of these applications are difficult to achieve due to cofactor inactivation in complex biological environments and cofactor-induced cellular stress. In this study, I explore the most recent strategies for developing robust, biocompatible ArMs that function in cell lysates, on cell surfaces or intracellularly. The pros and cons of developing and using ArMs in these three environments are described. I also examine how active ArMs might tolerate their environment, and the outstanding challenges and opportunities, including the need for simple methods of construction, improved catalytic performance and exploration of other reactions and microorganisms.</p><p></p>