This monograph provides researchers, engineers, postgraduates and lecturers working in the field of ferroelectric or piezoelectric and related materials with features of the structure-property relationships in modern piezo-active composites. These are piezoelectric composites which are active dielectric materials, which can be poled ferroelectric ceramics or domain-engineered single crystals poled along specific crystallographic directions. Current knowledge of the effective physical properties of these materials is lacking especially due to gaps of information in physical, chemical, microgeometric and technological factors. For composite and transducer design purposes, the expected properties of these piezo-active materials have been theorized through models by the authors and proven in experiments. Various well-known journals have published this research, among many others: Smart Materials and Structures; Journal of Physics D: Applied Physics; IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control; Acta Materialia. The book summarises and generalises a series of authors' works on the problem of the effective properties and related parameters of modern two- and three-component piezo-active composites wherein the microgeometric factor plays the dominating role. Specific examples of the performance of composites based on domain-engineered single crystals are also discussed. New trends are described in the research of modern piezo-active composites with the aim of filling the gaps in piezoelectric materials science. The primary goal of the book is to show advantages of different methods being applied to manufacture and study the functional composites that are suitable for piezoelectric energy harvesting, hydroacoustic, sensor, actuator, and other transducer applications.