Tailored-made biocatalysts that meet specific industrial or academic demands can be developed through enzyme engineering. By mutating one or several amino acids, the specificity or selectivity of enzymes can be altered to unlock novel and effective synthetic routes towards valuable compounds. Enzyme engineers typically target amino acids in the active site. But before reaching the active site, ligands first have to migrate through an access tunnel, and this passage can certainly contribute to specificity or selectivity as well. However, despite their great potential, the amino acids located in access tunnels are regularly ignored. In this project, I will investigate the evolution of two functionally unrelated enzyme families to study the underlying mechanisms of tunnel evolvability and the co-evolution between tunnel and active site. By drawing inspiration from natural evolution, fundamental knowledge gaps on the effect of tunnel mutations will be filled in and novel tunnel based engineering strategies will be developed. This will not only enlarge our knowledge on natural protein evolution, but also our protein engineering toolbox.