In this chapter, we will identify multiple concepts and principles that will prove essential for understanding fluid-like behaviors of continuous materials. A fluid can be defined as any substance that flows under the action of a shear stress, no matter how small the shear. Alternatively, some define a fluid as any substance that quickly assumes the shape of the container in which it is placed. Regardless of the specific definition, common experience shows that materials in either their liquid or their gaseous phases can fall within the purview of fluid mechanics.