This book describes the most general linear and local boundary conditions and analyses the basic problems of plane-wave reflection and matching associated to a planar boundary in a simple-isotropic medium. The book introduces the most general set of boundary conditions restricted by linearity and locality. Particular familiar forms of boundary conditions which have been covered over the years appear as special cases of the most general ones. The book starts with known special cases in Chapter 1 and, making generalizations, arrives at the most general form in Chapter 5. In Chapter 6 another set of boundary conditions, not restricted by linearity, is discussed. In Chapter 7, practical computations on wave scattering from obstacles defined by various boundary conditions, are considered.