Neutrons with different energies, and hence different wavelengths, from de Broglie's relationship, travel at different velocities:
λ = h/mv
where m is the neutron mass and v the neutron velocity.
Thus, for all neutrons created at the same time, the higher energy, shorter wavelength neutrons travel faster and hence arrive at the sample and subsequently at the detector, at an earlier time than the lower energy, longer wavelength, slower neutrons. By measuring the time of arrival of a neutron at the detector, and of course knowing its flight path, we can calculate its velocity and hence its wavelength (energy). This is the basis of TOF, which is a genuinely wavelength-sorted white beam technique of great use in a wide range of techniques (diffraction and inelastic neutron scattering).

Time-of-flight spectrometers may be divided into two classes:

- Direct geometry spectrometers: in which the incident energy is defined by a device such as a crystal or a chopper, and the final energy is determined by time-of-flight

- Indirect (inverted) geometry spectrometers: in which the sample is illuminated by a white incident beam and the final energy is defined by a crystal or a filter and the incident energy is determined by time-of-flight.

At a pulsed source all spectrometers use the time-of-flight techniques. On steady state sources pulsing devices such as choppers are required.