Understanding the quark structure of matter has been one of the most important advances in contemporary physics. It has unravelled a new and deeper level of structure in matter, and physics at that level reveals a unity and aesthetic simplicity never before attained. All forces emerge from a unique invariance principle and each of the basic interactions results from a specific symmetry property. Quarks interact among themselves through their "colour", as now accurately described by quantum chromodynamics.This volume brings together eight major review articles by Maurice Jacob, a physicist at the forefront of research on the quark structure of matter. He has, in particular, been involved with two research topics in this field. The first is the study of hadronic jets, which one actually sees instead of quarks, because of the opacity of the vacuum to colour. The second is the search for quark matter, a new form of matter believed to exist at high temperatures, when the vacuum should become transparent to colour.The papers in this volume provide a comprehensive review of these phenomenological studies on the quark structure of matter, and also a fasinating insight into the pace of recent progress in these areas. The book comes complete with an original introduction by the author, and also contains a pedagogical review on what is a most engrossing and rewarding field of research in physics.