Slowness versus depth functions offer a versatile complement to velocity versus depth functions. Obtaining the time-depth relationship, which is essential for time to depth conversion, is greatly facilitated by the convenient mathematical form of the slowness versus depth functions. This feature allows them to be used to provide an accurate description of a wide range of forms of slowness (hence, velocity) variation with depth. Examples of polynomial and asymptotic slowness versus depth functions are given. As in the case of velocity versus depth functions, the applicability of some function forms (such as high-order polynomials) may diminish rapidly outside the depth range over which the function is derived.