Book excerpt: In this volume new group-theoretical methods for investigating phase transitions and symmetry problems in various types of crystals and liquid crystals are presented. New topics pertaining to the area of application of the theories of Landau and Birman, such as, transitions to modulated structures and the description of their symmetry, are discussed. Examples of applications of the group-theoretical methods are given, containing detailed calculations of symmetry changes at structural, magnetic, improper ferroelectric, commensurate-to-incommensurate, field-induced metamagnetic and liquid crystal phase transitions. This book will be invaluable to research workers at universities and scientific laboratories and postgraduate students specializing in solid state theory.