This paper presents a new isolated-cell pressure-decay method for measuring ultralow permeabilities on unconventional rocks. The new method improves signal-to-noise ratio, increases the range of measureable permeability, reduces thermal effects, and improves pore volume to dead volume ratios compared to conventional methods using connected-cell pressure-decay systems as, for example, the Gas Research Institute (GRI) permeability method. Permeability, along with other properties such as porosity and grain volume, is inferred from interpretation of the full transient pressure-decay curves recorded during diffusion of gas into the sample, and not limited to early- or late-time approximations. The interpretation of pressure-decay measurements is based on a rigorous physical-mathematical model, which provides great flexibility for running and analyzing pressure-decay tests at different conditions. Flexibility in the size of the samples allows us to select the optimal size for best testing accuracy within a given testing time, if the samples are homogenous. If the samples are heterogeneous, testing multiple sample sizes provides insight into the distribution of permeability. The interpretation method also allows us to analyze pressure-decay curves recorded for either crushed material or controlled-shape fragments with known dimensions. Finally, the method allows us to record and interpret pressure-decay measurements at different pore pressures and therefore it provides information characterizing gas slippage or Knudsen flow effects. By recording the apparent pressure-dependent permeability and relating it to a theoretical model that defines permeability as a function of Knudsen number, it is possible to determine the characteristic width of interconnected porosity. The latter provides unique information about the geometry of the pore space participating in the flow and controlling the properties of the flow. Other methods, such as gas sorption, mercury intrusion porosimetry, and scanning electron microscopy, generate information on pore size distribution and pore space geometry. However, these methods do not predict the relative contribution of all different pores into the overall flow. The effective width of the channels that are actually controlling the flow represents an important metric that helps us to characterize the porous medium and allows proper extrapolation of laboratory data to understand and predict flows of in-situ fluids under reservoir conditions.