Hydrodynamical equations are usually derived from the Boltzmann equation. In this chapter, however, assuming continuous hydrodynamical systems from the beginning, we summarize the basic set of hydrodynamical equations. There are two ways to describe hydrodynamical motions. One is the Eulerian description and the other is the Lagrangian one. Each one has merits and demerits. In this chapter both descriptions are presented, and basic characteristics of fluid motions described by these descriptions are summarized. A derivation of hydrodynamical equations from the Boltzmann equation is made in Chap. 12 as a part of derivation of hydromagnetic fluid equations.