In this talk, I will discuss how chemical design can be used to create single-molecule circuits with functional properties. I will first explain from a theoretical perspective how quantum interference effects can be exploited to create circuits with highly nonlinear current-voltage characteristics. I will then show STM-break junction measurements for a series of benzothiadiazole (BT)-based molecular wires. The electron withdrawing BT unit localizes the lowest unoccupied molecular orbital, thereby enhancing destructive quantum interference and enabling circuits with large dynamic range. The conductance of a 6-nm BT wire can be modulated by a factor of more than 10,000 using this design strategy. Finally, I will introduce a modified STM technique that employs impedance spectroscopy to interrogate the solvent environment surrounding single-molecule circuits. Solvent-induced shifts in molecular conductance can be correlated to changes in capacitance at the metal-solvent interface. Together, these results provide a framework for chemical control of charge transport at the nanoscale.