Temperature and heat, entropy and order or disorder are key classical concepts of physics. These are challenged by searching matter under extreme conditions, such as high (relativistic) energy, strong acceleration or gravitation, or unusual complexity due to long range correlations. In our quest for quark matter all these conditions might occur simultaneously. This book, strongly motivated by the authors' everyday research experiences in the field of high-energy heavy-ion collisions, aims to bundle these challenges to modern physics. The main topic is at the heart of thermodynamics -- the very.