An account is given of the theoretical basis for the design of coaxial resistors that retain their d.c. resistance, without appreciable reactance, at all frequencies at which coaxial systems are normally used. The main restriction on physical size is governed by the avoidance of supplementary modes of propagation. The design employs a uniform cylindrical film resistor with a critically dimensioned outer conductor, the profile of which has the form of a tractrix. Lead-in cones are designed to avoid discontinuity at the connections with the resistor and the outer conductor. Experimental results show an impedance within 1% of the d.c. resistance, with an extremely small phase angle, at all frequencies up to the highest measured, namely 3 450 Mc/s.