There are many test methods available to determine the pore sizes of geotextile filters. Often overlooked is the fact that each method provides a different interpretation of a geotextile's pore structure. A survey of the available test methods, indicates that four categories of pores can be measured. Pore characteristics are identified by one of the following methodologies: sieving, theoretical, volumetric, and numerical. A critical review of the data produced by these methods shows that the actual pore structure as related to flow-through behavior can only be determined by the bubble point method, which provides a volumetric pore size. This test identifies the constriction size of each pore channel in a geotextile, where retention and clogging are most critical. The bubble point test method is advantageous because it provides for the complete pore size distribution of the geotextile, can be performed more efficiently than sieving methods, and provides an accurate estimate of the permeability of the geotextile.