Characterization of Inverse Problems.- Discretization of Inverse Problems.- Regularization of Linear Inverse Problems.- Regularization of Nonlinear Inverse Problems.- Appendix A. Results from Linear Algebra.- Appendix B. Function Spaces.- Appendix C. The Fourier Transform.- Appendix D. Regularization Property of CGNE.- Appendix E. Existence and Uniqueness Theorems for Waveform Inversion.