An exposition of the principles of straight-line diode detection is given with reference to the direct production of an undistorted audio-frequency output voltage from a modulated radio-frequency input voltage. The exposition is accompanied by experimental evidence, which establishes the validity of the theory.It is shown that by proper choice of circuit constants, almost perfect detection can be obtained, even with 100 per cent modulation. Both the theory and the experiments take account of the circuit coupling the detector to the next valve. The effect of positive bias is examined and is shown to be beneficial.