Clay minerals are the most abundant materials in sedimentary basins. The most common—like kaolinite, illite, chlorite, and smectite—are found in various amounts in mudstones and are also often found in clastic and nonclastic reservoir rocks. Their presence alters the elastic behavior of reservoir rocks significantly as a function of mineral type, volume and distribution. Thus, two sandstones with the same clay amount might have different elastic properties due to differences within the clay population. The elastic properties of clay minerals are therefore important in rock physics modeling to understand the seismic and sonic log responses of shaley sequences and clay-bearing reservoir rocks.