  As a consequence of a general trend in the physics of oscillators and clocks
towards optics, phase and frequency metrology is rapidly moving to optics too.
Yet, optics is not replacing the traditional radio-frequency (RF) and microwave
domains. Instead, it adds tough challenges. Precision frequency-stability
measurements are chiefly based on the measurement of phase noise, which is the
main focus of this article. Major progress has been achieved in two main areas.
The first is the extreme low-noise measurements, based on the bridge
(interferometric) method in real time or with sophisticated correlation and
averaging techniques. The second is the emerging field of microwave photonics,
which combines optics and RF/microwaves. This includes the femtosecond laser,
the two-way fiber links, the noise measurement systems based on the fiber and
the photonic oscillator. Besides, the phenomenology of flicker (1/f) noise is
better understood, though the ultimate reasons are still elusive.
