In various applications, we can meet boundary value problems for elliptic equations whose ellipticity is disturbed in the sense that some degeneration or singularity appears. This bad behavior can be caused by the coefficients of the corresponding differential operator as well as by the solution itself. There are several very concrete problems form practice which lead to such differential equations, e.g. from glaciology, non-Newtonian fluid mechanics, flows through porous media, differential geometry, celestial mechanics, climatology, reaction-diffusion problems, etc. This book is based on research to the author on degenerate elliptic equation. This book will be a useful reference source for graduate students and researches in differential equations.