Abstract: The invention relates to systems and methods for engineering, testing or modelling a biological circuit. The biological circuit is split into two parts, wherein one part is implemented in one or more living cells and the other part is simulated on a computer and the two parts interface via a closed loop in real time. The system comprises (i) means for culturing one or more living cells; (ii) one or more living cells in which a cellular part of the biological circuit is implemented; (iii) means for controlling the state or environment of the cell(s); (iv) means for taking measurements of one or more cell state or environmental parameters from the cell(s); and (v) a computer.