An aquifer is a water-saturated porous medium composed of geologic unit(s) that can store and transmit signifi cant quantities of water under ordinary conditions. The term reservoir has the same general meaning, but it is often used when the fl uid of interest is petroleum. In this text, the primary focus is the assessment of soils and unconsolidated sediments comprising aquifer systems. However, bedrock aquifer systems consisting of consolidated sediments, igneous, and metamorphic rocks also contain interconnected pore spaces due to depositional and genetic processes, as well as physical and chemical weathering. In unconsolidated sediments and soils the characteristics of the matrix skeleton are a direct result of the depositional environment, which dictates the type and volume of the sediment, the shapes of the mineral grains, and distribution of particle grain sizes. The more dynamic the nature of the depositional process that created the deposit, the higher the degree of variability to be expected in the grain sizes composing the skeletal matrix. The depth of emplacement and loading history subsequent to deposition also affect the density and packing of the skeletal matrix. To this end, we recommend a multidisciplinary approach to aquifer characterization that incorporates quantitative geology and hydrogeology, as well as engineering methods.