In physics, the fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four conventionally accepted fundamental interactions-gravitational, electromagnetic, strong, and weak. Each one is described mathematically as a field. The gravitational force is modelled as a continuous classical field. The other three, part of the Standard Model of particle physics, are described as discrete quantum fields, and their interactions are each carried by a quantum, an elementary particle. The strong and weak interactions have short ranges, producing forces at minuscule, subatomic distances; these forces govern nuclear interactions. The strong interaction, which is carried by the gluon particle, is responsible for the binding of quarks together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction, which is carried by the W and Z particles, also acts on the nucleus, mediating radioactive decay. The other two, electromagnetism and gravity, produce significant forces at macroscopic scales where the effects can be seen directly in everyday life. The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for chemical bonding and are used in electrical technology. Electromagnetic forces tend to cancel each other out when large collections of objects are considered, so over the largest distances (on the scale of planets and galaxies), gravity tends to be the dominant force. All four fundamental forces are believed to be related, and to unite into a single force at high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. A goal of theoretical physicists working beyond the Standard Model is to quantize the gravitational field, yielding a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Other theorists seek to unite the electroweak and strong fields within a Grand Unified Theory (GUT). Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE). A few researchers have interpreted various anomalous observations in physics as evidence for a fifth force, but this is not widely accepted. This book is designed to be a state of the art, superb academic reference work and provide an overview of the topic and give the reader a structured knowledge to familiarize yourself with the topic at the most affordable price possible. The accuracy and knowledge is of an international viewpoint as the edited articles represent the inputs of many knowledgeable individuals and some of the most current knowledge on the topic, based on the date of publication.