This book gives a modern presentation of spectral and scattering theory for Sturm-Liouville equations, including results on inverse theory. One of the distinctive features of the book is the in-depth treatment of left-definite Sturm-Liouville equations. \par Chapter 1 discusses an elementary approach to regular Sturm-Liouville equations based on variational methods. \par Chapter 2 gives a brief overview of operator theory for bounded and unbounded operators as well as relations. \par Chapter 3 develops the general spectral theorem for unbounded self-adjoint operators and the associated functional calculus. \par Chapter 4 discusses the spectral theorem and eigenfunction expansions for regular and singular Sturm-Liouville equations. \par Chapter 5 develops a spectral theory for left-definite Sturm-Liouville equations including eigenfunction expansions and does this under minimal regularity assumptions on the coefficients. \par Chapter 6 discusses Sturm's classical results on zeros of solutions and develops basic results on spectral asymptotics for right- and left-definite Sturm-Liouville equations. \par Chapter 7 presents uniqueness theorems of inverse spectral theory for right- and left-definite Sturm-Liouville equations. \par Chapter 8 is devoted to scattering theory for right and left-definite general Sturm-Liouville equations on the real line. The scattering and inverse scattering theory for the left-definite case is of importance to the Camassa-Holm equation. \par The main text is supplemented with an extensive appendix containing necessary background material on functional analysis, integration theory, distribution theory, ordinary differential equations, analytic functions, and the Camassa-Holm equation.