The facility with which water, as well as other fluids, will move through, or be held in or drawn into, the pores of soils and rocks is very significant in many types of engineering, hydrologic, agricultural, and geologic problems. Permeability and capillarity that measure this facility thus represent some of the most important properties of soils and rocks and the determination of these properties is a most important aspect of the testing of these materials. These properties play a vital part in problems related to drainage of highways and agricultural lands, seepage through earth dams and levees, uplift pressure beneath concrete dams and other construction, estimating available groundwater storage, dewatering construction sites, moisture distribution above the groundwater table, recovery of petroleum from oil-bearing strata, groundwater recharge by wells or spreading basins, disposal of atomic and other wastes by well injection, and seepage pressures that cause earth slides. Although the permeability and capillarity tests of rocks and soils are quite simple in theory, many factors affect such tests, both in the field and in the laboratory. Thus, the movement of water through soils and rocks, whether driven by pressure or capillary forces, is immensely complicated. The testing engineer must be aware constantly of the many factors that may affect the results obtained from tests for permeability and capillarity. The empirical law discovered in 1856 by Darcy provides us with a fundamental law for flow of water through soils and rocks. This law has been extensively tested, and its validity apparently has been established under a wide variety of conditions. A knowledge of Darcy's law is essential to an understanding of the flow of water through soils and rocks and for analysis of the many problems mentioned previously. ASTM Committee D-18 on Soils and Rocks for Engineering Purposes has responsibility for developing methods of testing soils and rocks and for furthering research activity in the general field of properties and behavior of soils and rocks for engineering purposes. The members of Committee D-18, however, are very much aware that natural materials such as soil and rock cannot be treated like most controlled materials. Thus, quality soil and rock testing never should be a simple routine matter. As indicated by Holtz, soil and rock must be evaluated in terms of their past history, constituent parts, the treatment comtemplated, and the conditions to which they will be subjected. Thus, some details of soil and rock testing procedures must be flexible to take into account these factors, and experiment and judgment of a high order are demanded from the testing engineers.