This book aims to provide an explanation of the scientific basis and the methodologies adopted in the implementation of the OpenQuake engine, an open source code for seismic hazard and physical risk calculation. The book follows the traditional openness and transparency features of the Global Earthquake Model (GEM) as clearly indicated in the development principles of the OpenQuake engine. The GEM initiative is a global collaborative effort with the aim to provide organisations and people with tools and resources for transparent assessment of earthquake risk anywhere in the world. The OpenQuake engine is a fully integrated, flexible and scalable hazard and physical risk calculation engine whose development is at the core of GEM’s overall objectives.