This set of six papers, written by eminent experts in the field, is concerned with that part of fluid mechanics that seeks its foundation in the rigorous mathematical treatment of the Navier-Stokes equations. In particular, an overview is given on state of research regarding the global existence of smooth solutions, for which uniqueness and continuous dependence on the data can be proven. Then, the book moves on to a discussion of recent developments of the finite element Galerkin method, with an emphasis on a priori and a posteriori error estimation and adaptive mesh refinement. A further article elaborates on spectral Galerkin methods and their extension to domains with complicated geometries by employing the techniques of domain decomposition. The rigorous explanation of bifurcation phenomena in fluids has long been a central topic in the theory of Navier-Stokes equations. Here, bifurcation theory is introduced in a general setting that is particularly convenient for application to such problems. Finally, the extension of Navier-Stokes theory to compressible viscous flows, studied in two more papers, opens up a fascinating panorama of theoretical and numerical problems. While some of the contributions are expository, others primarily present new results within a wider context and fuller exposition than is usual for research papers. The book is meant to introduce researchers and advanced students to the research level on some of the most important topics of the field.