The optical fiber used as a microwave delay line exhibits high stability and low noise and makes accessible a long delay $(\gtrsim 100\mu \mathrm{s})$ in a wide bandwidth ( $\approx 40\mathrm{GHz}$ , limited by the optronic components). Hence, it finds applications as the frequency reference in microwave oscillators and as the reference discriminator for the measurement of phase noise. The fiber is suitable to measure the oscillator stability with a sensitivity of parts in ${10}^{-12}$ . Enhanced sensitivity is obtained with two independent delay lines, after correlating and averaging. Short-term stability of parts in ${10}^{-12}$ is achieved inserting the delay line in an oscillator. The frequency can be set in steps multiple of the inverse delay, which is in the $10–100\mathrm{kHz}$ region. We add to the available references a considerable amount of engineering and practical knowledge, the understanding of $1∕f$ noise, calibration, the analysis of the cross-spectrum technique to reduce the instrument background, the phase-noise model of the oscillator, and the experimental test of the oscillator model.