The problem we are examining here is that of building a mathematical model that simulates the behavior of a fluid, which can be a gas or a liquid. In this analysis, Thermodynamics joins Mechanics with a greater role than that usually performed in the case of solids. We immediately point out that, even in the presence of thermal effects, the analysis of the main problems of engineering interest can be conducted by simulating the body with a continuous medium, in the mathematical sense. However, we specify that in some engineering applications of Fluid Dynamics we encounter fluids for which the continuum model is not admissible (as in the case of rarefied gases). In fluid dynamics, as a rule, non-negligible mechanical energy dissipation phenomena occur, with consequent production of entropy. If entropy is produced during a transformation, this cannot be canceled and it follows that the transformation considered is irreversible. It is therefore obligatory, in order to construct an adequate model of the problem, to resort to the Thermodynamics of irreversible processes. However, let us premise some references to classical Thermodynamics, based on a theory formulated with postulates. This ebook is an introduction to Fluid Dynamics. It deduces its fundamental results favoring (when possible) considerations of a physical nature. It discusses in detail: the equations of fluid dynamics the characteristic numbers (of Strouhal, of Weber, of Mach, of Reynolds, of Prandtl, of Froude, of Peclet) the motion of a gas in a convergent-diverging plane nozzle dissipative motions the laminar regime the turbulent regime.