1. Classical hydrodynamic stability.- 2. Generalized solutions in hydrodynamic stability.- 3. Branching and stability of the solutions of the Navier-Stokes equations.- 4. Nature of turbulence.- 5. The influence of the presence of a porous medium on hydrodynamic stability.- Appendices 1. Operators in Hilbert spaces.- 2. Semigroups of operators in Banach spaces.- 3. Spectral theory of linear operators.- 4. Calculus of variations.- 5. Geometric methods in branching theory.- 6. New methods for solving the Orr-Sommerfeld equation.- 7. Analytical methods to solve some eigenvalue problems in hydrodynamic and hydromagnetic stability theory.- 8. Stability of nonstationary fluid flows.- Afterword.