In 2006, "Or-Nano", a French scientific network on gold nanoparticles was created in France. The research group saw the need for a gathering of specialists on gold nanoparticles to facilitate deeper scientific exchange between the different scientific communities. The result is this book which provides a broad introduction to the fascinating and beautiful world of gold nanoparticles. This volume traces the fascinating history of gold nanoparticles which began in ancient times with red ruby glass, and reached its zenith at the end of the 17th century. It also provides an in-depth investigation into the use of gold nanoparticles over the ages - an important aspect that is almost never treated in scientific books. Each chapter provides a lucid overview of the different aspects of gold nanoparticles including synthesis, optical and thermal properties, interaction with surfaces, catalysis, reactivity, biosensors, and nanomedicine. There is an ample focus on promising fields where gold nanoparticles are already creating breakthroughs such as in catalysis, cancer therapy and plasmonics. The issue of toxicity is also discussed comprehensively. The work may be used as an advanced textbook by graduate students and young scientists who need an introduction to gold nanoparticles. It will also be suitable for experts in the related areas of chemistry, biology, material science, and physics, who are interested to broaden their knowledge and provide an overview of the subject. The explanations in the book are detailed enough to capture the interest of the reader, and complete enough to provide the necessary background material needed to delve further into the subject and explore the literature.