An apparatus for measuring the permeability of a core sample. Measurements are made either by placing the sample in a chamber (also known as a core holder), or by placing a probe on the surface of the sample. Core-holder measurements are made either with gas or liquid, and either in steady state or unsteady-state conditions. Other variables include the confining pressure and the direction of measurement, which can be axial (along the axis of a cylindrical core sample), transverse (perpendicular to the axis), or radial (to the center of a hollow cylinder). In probe measurements, gas is injected into the sample under either steady- or unsteady-state conditions. Probe permeameters are also known as minipermeameters.