  The changes in brightness of an astronomical source as a function of time are
key probes into that source's physics. Periodic and quasi-periodic signals are
indicators of fundamental time (and length) scales in the system, while
stochastic processes help uncover the nature of turbulent accretion processes.
A key method of studying time variability is through Fourier methods, the
decomposition of the signal into sine waves, which yields a representation of
the data in frequency space. With the extension into \textit{spectral timing}
the methods built on the Fourier transform can not only help us characterize
(quasi-)periodicities and stochastic processes, but also uncover the complex
relationships between time, photon energy and flux in order to help build
better models of accretion processes and other high-energy dynamical physics.
In this Chapter, we provide a broad, but practical overview of the most
important relevant methods.
