Transition-metal catalysis has changed the way in which chemical reactions can be accomplished. While most metal-catalyzed reactions have been achieved in organic solvents, recent work has demonstrated that many of these transformations can be made compatible with water. These discoveries have stimulated the search for metal catalysts that are capable of achieving designed reactions in biological settings, and eventually behave as non-natural enzymes working in native cellular environments. Although this new field of research is still taking its first steps, there is a growing number of publications in the area, and one can predict that it will steadily grow in the years to come. Here we will briefly review some of the main contributions in the area. The contents have been organized according to the type of transformation and transition metal catalysts involved in the process.