The intention of the book is to highlight the problematic aspects of turbulence. The contributions treat a variety of mathematical, physical and engineering subjects related to turbulence. The topics include mathematical issues, control and related problems, observational aspects, two- and quasi-two-dimensional flows, basic aspects of turbulence modeling, statistical issues and passive scalars. The main questions addressed are the controllability of turbulent flows, possible qualitative differences between pure two-dimensional and real quasi-two-dimensional turbulent flows, common features of two-dimensional and three-dimensional turbulence, universality (or not) of small-scale turbulence and its relation to large scales, the question of how realistic the prospects of reduced description of turbulent flows are, the necessity of dealing more with the physics of turbulence in general, and in turbulence modeling and (beyond) scaling properties, in particular.