This lecture introduces the various geologic formations that can serve as aquifers, emphasizing the importance of understanding their specific hydrogeologic characteristics. Groundwater is found in diverse geological settings, from unconsolidated alluvium to fractured granite, each with distinct storage and flow properties. The ability of a rock to act as an aquifer depends on both its porosity, which affects water storage and its permeability, which influences water flow. For example, clay may hold significant water but have low permeability, while fractured granite may be highly permeable but store less water. Overall, the lecture highlights how different types of rocks and sediments impact groundwater availability and movement.