The recent development of exact methods for determining the phase in neutron specular reflectivity measurements makes it possible to extract the scattering length density (SLD) depth profile of a film directly, by first principles inversion. Ideally, other than the uncertainty introduced by statistical fluctuations in the measured reflectivities, any ambiguity in a SLD profile obtained by these phase-sensitive methods is a consequence only of the limited range of wavevector transfer Q over which the reflectivity data can, in practice, be collected. Where practically possible to employ such phase-sensitive techniques, which require the use of reference layers or variable surrounding media, not only is the uniqueness of the resultant SLD profile well defined, but also the ability to perform an inversion without any adjustable parameters eliminates the vagaries associated with fitting reflectivity data alone. A comprehensive summary of the experimental and theoretical methodologies of phase-sensitive neutron reflectometry is presented and illustrated by a current application.