A quark is a type of elementary particle found in protons and neutrons and other subatomic particles. They are a major constituent of matter, along with leptons. In nature, quarks are never found on their own, as isolated, single particles; rather, they are bound together in composite particles named hadrons. For this reason, much of what is known about quarks has been inferred from observations on the hadrons themselves. This book provides leading edge research on this field from around the globe.