The factorization method is a relatively new method for solving certain types of inverse scattering problems in tomography. Aimed at students and researchers in Applied Mathematics, Physics, and Engineering, this text introduces the reader to this promising approach for solving important classes of inverse problems. The wide applicability of this method is discussed by choosing typical examples, such as inverse scattering problems for the scalar Helmholtz equation, a scattering problem for Maxwell's equation, and a problem in impedance and optical tomography. The last section of the book compares the Factorization Method to established sampling methods (the Linear Sampling Method, the Singular Method, and the Probe Method)