The recent discovery of the tunnel diode with band-widths extending into the kilomegacycle region has prompted investigation of their use in the logic and control portions of high-speed computers. Considerations of diode uniformity requirements, stability problems and power supply requirements has led to a monostable type of logical circuit. The switching properties of this circuit are analyzed and found to depend upon the negative resistance-capacitance time constant of the unit. The basic function performed by the circuit is a thresholding operation from which a set of logical building blocks is derived. Compatible dynamic and bistable storage schemes are discussed. Of major importance is the effect of diode variations upon the logical gains and delays of the circuits. These properties have been tabulated for tunnel diodes with 5 per cent tolerances on knee current and voltage. Experimental circuits using diodes with a time constant of 1.4 nanoseconds have given a nominal switching time of 7.5 nanoseconds.