Enzymes are biology's tiny chemists. They help build, break and rearrange molecules quickly and precisely. In the lab, scientists have long put enzymes to work making useful chemicals. But nature's enzymes cannot do every reaction a researcher might want. Sometimes scientists need an enzyme to take on a job it did not evolve to do. That's where enzyme engineering comes in—researchers make changes to an enzyme, test how those changes affect what the enzyme does and look for versions that work better.