This book is an in-depth review of experiment and theory on electric-dipole polarizabilities. The book is broad in scope: it encompasses atomic, molecular and cluster polarizabilities. Both static and dynamic polarizabilities are treated (in the absence of absorption) and a full tensor picture of the polarizability is used. Traditional experimental techniques for measuring electric polarizabilities are described in detail. Recently-developed experimental methods, including light-forces, position-sensitive time-of-flight deflection, and atom-interferometry are also extensively discussed. Theoretical techniques for calculating polarizabilities are reviewed, including a discussion of the use of Gaussian basis sets. Many important comparisons between theory and experiment are summarized in an extensive set of tables of polarizabilities of important atoms, molecules and clusters. Applications of polarizabilities to many areas of chemistry and physics are described including: optics, chemical structure, interactions of gases and particles with surfaces, and the interaction of molecules with light. The emphasis is on a lucid presentation of the ideas and results with up-to-date discussions of important applications such as optical tweezers and nanostructure fabrication. The book provides an excellent overview of the importance of polarizabilities in understanding the physical, electronic and optical properties of particles in a regime that goes from free atoms to condensed-phase clusters.