This paper is an account of an attempt to perform logic at very high speed (250 MHz) using tunnel diodes. The basic circuit, a derivative of the Goto pair, is described and the rather limited logic power of such an element defined. An account is given of a suitable application for a small amount of very high speed logic, namely a variable microprogram control unit for a transistor digital computer. A brief description of the incorporation of the tunnel diodes in the machine follows.The microwave diodes used were found to be too unreliable for largescale use in a digital computer. By careful selection from a large number (5000) of diodes, 700 logic elements were constructed and interconnected, to confirm the circuit analysis and logic rules. It is concluded that in the event of reliable microwave tunnel diodes becoming available in large numbers they could be used in the described manner to perform logic at 250–300 megabit rates and 1 ns stage delays.