Existence, Uniqueness, and Regularity of Solutions.- Semidiscrete Discretization.- Convolution Quadrature.- Finite Difference Methods: Construction and Implementation.- Finite Difference Methods on Uniform Meshes.- Finite Difference Methods on Graded Meshes.- Nonnegativity Preservation.- Discrete Fractional Maximal Regularity.- Subdiffusion with time-dependent coefficients.- Semilinear Subdiffusion Equations.- Time-Space Formulation and Finite Element Approximation.- A Spectral Petrov-Galerkin Method.- Incomplete Iterative Solution at the Time Levels.- Optimal Control with Subdiffusion Constraint.- Backward Subdiffusion Problems.- Appendix: Mathematical Preliminaries.