This graduate-level textbook deals with different aspects of plane mirrors and mirror-related symmetries. It provides us with some new ways of understanding symmetries in crystals and the mirror combination schemes. The inclusion of topics such as the Wigner–Seitz unit cell, reciprocal lattice, Brillouin zones, diffraction of crystals, etc., based on the mirror combination scheme, are extremely helpful in understanding many other concepts in crystallography. A mirror is the only fundamental symmetry in crystals, and all other permissible symmetries in crystalline solids can be derived from suitable combinations of mirrors, called derived symmetries. A rudimentary knowledge of symmetry in crystallography is essential to students, researchers, and professionals in many subjects in science and technology: physics, chemistry, mathematics, molecular biology, geology, metallurgy, and particularly materials science and mineralogy.