This chapter covers the basic principles of soil characteristics, groundwater occurrence, and soil and groundwater contamination assessment and monitoring. Groundwater is mostly stored in an unconsolidated soil layer. The amount of water a soil layer can hold is dependent on the soil grain size, sorting, and packing. Laws and principles governing groundwater movement can be developed from the fundamental laws of physics. The direction and movement of groundwater in porous media are determined by areas of recharge and discharge and the system of interconnected passages comprising the microstructure of the porous matrix. Groundwater monitoring well design and construction involve several steps, including selection of target zone; selection of screen length, screen slot size, and filter pack; well installation; and well development. Gauging a well is completed prior to sampling to determine the depth to the bottom of the casing, the depth to the water surface, and the thickness of free product.