Semi-inner products, that can be naturally defined in general Banach spaces over the real or complex number field, play an important role in describing the geometric properties of these spaces. This new book dedicates 17 chapters to the study of semi-inner products and its applications. The bibliography at the end of each chapter contains a list of the papers cited in the chapter. The interested reader may find more information on the subject by consulting the list of papers provided at the end of the work. The book is intended for use by both researchers and postgraduate students interested in functional analysis. It also provides helpful tools to mathematicians using functional analysis in other domains such as: linear and non-linear operator theory, optimization theory, game theory or other related fields.