This book is a second completely revised edition of An Informal Introduction to Turbulence. The main emphasis is on conceptual and problematic aspects, physical phenomena, observations, misconceptions and unresolved issues rather than on conventional formalistic aspects, models, etc. Apart from the obvious fundamental importance of turbulent flows such an emphasis is a consequence of the view that without corresponding progress in fundamental aspects there is little chance for progress in any applications such as drag reduction, mixing, control and modeling of turbulence. More generally there is a desperate need for physical fundamentals of the technological processes in which turbulence plays a central role.The conceptual issues are made dominant in this second edition. This required to address in more detail those misconceptions which are the consequence of the profound difficulties of the subject and which travel from one publication to another. A new chapter titled Analogies, misconceptions and ill defined concepts was added along with a number of new sections on such topics as ergodicity, Eulerian versus Lagrangian descriptions, on validation of theories, on anomalous scaling and ill posedness of the concept of inertial range, on the Tennekes and Lumley balance, and mathematics versus turbulence among others. Many of these are to a large extent a consequence of the series of lectures delivered in the Imperial College London in 2007 and 2008 in the frame of Marie Curie Chair in Fundamental and Conceptual Aspects of Turbulent Flows which was held by the author during the period June 1, 2006 May 31, 2009.