Cells are living machines operated by a web of genetic, chemical and electrical circuitry. Until recently, our primary way of understanding cells was reverse engineering—breaking the cell one part at a time to infer its function. Now, building off advances in synthetic biology, it is becoming possible to also perform forward engineering—adding functions to the cell that do not yet exist. Building new functions in the cell will provide insights into the regulatory architecture, dynamics, and evolution of existing cells and open new avenues in biotechnology by programming cells to behave in useful ways.