An elaborate symphony is orchestrated in every living cell. The score is written in DNA, and is played out by RNA and protein enzymes, but where is the conductor of this symphony? Research in molecular biology over the past half century suggests that it is the intricate biochemical circuits (gene regulatory networks, cell signalling pathways, etc.) that play this role. In this context, it may be of value to learn to read and design the logic of biochemical circuits. I will give a tutorial-style introduction to the mathematics of chemical reaction networks, with a special emphasis on mass action kinetics.