The Finite Difference Method -- Finite Difference Calculus -- Elliptic Equations -- Iterative Methods for Elliptic Equations -- Parabolic Equations -- Hyperbolic Equations -- The Finite Element Method -- General Principles -- A 1-D Tutorial -- Multi-Dimensional Elements -- Time-Dependent Problems -- Vector Problems -- Numerical Analysis -- Inverse Methods -- Inverse Noise, SVD, and Linear Least Squares -- Fitting Models to Data -- Dynamic Inversion -- Time Conventions for Real-Time Assimilation -- Skill Assessment for Data Assimilative Models -- Statistical Interpolation. This book concerns the practical solution of Partial Differential Equations (PDEs). It reflects an interdisciplinary approach to problems occurring in natural environmental media: the hydrosphere, atmosphere, cryosphere, lithosphere, biosphere and ionosphere. It assumes the reader has gained some intuitive knowledge of PDE solution properties and now wants to solve some for real, in the context of practical problems arising in real situations. The practical aspect of this book is the infused focus on computation. It presents two major discretization methods - Finite Difference and Finite Element. The blend of theory, analysis, and implementation practicality supports solving and understanding complicated problems. It is divided into three parts. Part I is an overview of Finite Difference Methods. Part II focuses on Finite Element Methods, including an FEM tutorial. Part III deals with Inverse Methods, introducing formal approaches to practical problems which are ill-posed.