An electron-spin-resonance spectrometer is described which has a performance comparable with the best superheterodyne spectrometers but which avoids their complexity. It is ideally suited for electron nuclear double resonance (endor). The spectrometer was developed for use at Xband, but the principle of operation is applicable to any microwave-frequency band. The spectrometer operates by amplitude modulating at 460 kc/s the microwave signal reflected from the cavity. A high-gain a.f.c. system is required if optimum performance is to be obtained, and a system is described which allows the klystron frequency to be locked to the sample cavity with an incident microwave power of less than 0.5μW.The spectrometer can be operated with or without low-frequency magnetic-field modulation; in the absorption-derivative mode the sensitivity is approximately 5 × 1011 spins/Gs, and in the absorption mode the sensitivity is better than 1014 spins/Gs.