Enzymes are the only biomolecules that combine three hallmark features, namely—catalysis, specificity, and regulation. No wonder that the study of enzymes will continue to occupy the prime position in modern biology (and chemistry) (Editorial, Closing in on catalysis 2009). This much is amply obvious from each and every context and examples that we have come across in the preceding chapters. The study of enzymes in isolation—most often in purified form—has occupied much of the time in the past. Importance of enzyme function in vivo will form one of the key frontiers with the present emphasis on systems biology as a pointer in this direction. Enzymes in sequence, in combination with other enzymes and other cellular components bring in interesting features often not manifested by an enzyme in isolation—coupled reactions, regulatory networks, and distributed control of metabolism are some of them. While single enzyme studies have taken the back seat, the knowledge gleaned from such research is important for a systems biology approach. After all, any biologist who follows their research interest to the finest level of detail will become an enzymologist (quote by Perry Frey).