Understanding the Origin of Matter : Perspectives in Quantum Chromodynamics This book aims at providing a solid basis for the education of the next generation of researchers in hot dense QCD (Quantum ChromoDynamics) matter. This is a rapidly growing field at the interface of the smallest i.e. subnuclear physics and the largest scales namely astrophysics and cosmology. The extensive lectures presented here are based on the material used at the training school of the European COST action THOR (Theory of hot matter in relativistic heavy ion collisions). The book is divided in three parts covering ultrarelativistic heavy ion collisions several aspects related to QCD and simulations of QCD and heavy ion collisions. The scientific tools and methods discussed provide graduate students with the necessary skills to understand the structure of matter under extreme conditions of high densities temperatures and strong fields in the collapse of massive stars or a few microseconds after the big bang. In addition to the theory the set of lectures presents hands on material that includes an introduction to simulation programs for heavy ion collisions equations of state and transport properties. Introduction Part I Ultrarelativistic Heavy Ion Collisions Probing the QCD phase diagram with heavy ion collision experiments The early stage of heavy ion collisions Hydrodynamic description of ultrarelativistic heavy ion collisions Part II Aspects of Quantum Chromodynamics Three lectures on QCD phase transitions Effective approaches to QCD Heavy flavors and exotic hadrons Part III Simulations of QCD and Heavy Ion Collisions Flavored aspects of QCD thermodynamics from Lattice QCD Spectral and transport properties from lattice QCD Monte Carlo statistical hadronization in relativistic heavy ion collisions Index.