Inverse problems are those where, from ""external"" observations of a hidden ""black box"" system (a patient's body, nontransparent industrial object, interior of the Earth, etc.), one needs to recover the unknown parameters of the system. A prototypical example is the by now classical Calderon problem, forming the basis of Electrical Impedance Tomography (EIT). In EIT one attempts to determine the electrical conductivity of a medium by making voltage and current measurements at the boundary. EIT arises in several applications, including geophysical prospection and medical imaging. Since the original work of Calderon there has been remarkable progress on this problem. This textbook is an introduction to the mathematical theory of the Calderon problem. It includes a thorough account of many important developments. The book is intended for graduate students who are familiar with basics of real, complex, and functional analysis. The text can be used for short or long graduate level courses on this topic. Basic properties of weak solutions of partial differential equations, of Sobolev spaces, and of Fourier transform are developed in the text and appendices. Comprehensive Notes sections with further references to the literature will be helpful for those readers who wish to study this topic further.