The book is devoted to flexoelectricity, which is the response of polarization to strain gradient. The effect is analogous to piezoelectricity which is the response of polarization to strain itself. In contrast to piezoelectricity, which appears only in non-centrosymmetric materials, flexoelectric effect is allowed in any materials. It becomes appreciable in nano-scale systems, where large strain gradients arise. The book focuses on different nano-scale manifestations of the effect. It covers large number of manifestations ranging from repulsion of acoustic and optic branches of materials to flexoelectricity in domain walls. The book is mainly catered for scientists, engineers and students.