This second edition to A First Course in Partial Differential Equations provides a clear, rigorous, and student-friendly introduction to the core theory and solution techniques for partial differential equations (PDEs), making it an ideal text for upper-level undergraduates in mathematics, physics, engineering, and the applied sciences.This volume builds on the strengths of the first edition by integrating new topics that bridge classical theory with modern applications. In addition to comprehensive treatments of standard second-order linear PDEs -- the heat equation, wave equation, and Laplace's equation -- this edition includes substantial new content: Core topics also include first-order linear and nonlinear PDEs arising in the physical and life sciences, Fourier series, Sturm-Liouville problems, and special functions of mathematical physics. Appendices review essential background in complex analysis and linear algebra, ensuring accessibility for students from a broad range of STEM disciplines.With its flexible structure, this textbook supports both one- and two-semester courses, and provides a solid foundation for students preparing for graduate-level PDE courses. It is equally valuable as a reference text for researchers and practitioners seeking practical methods for solving PDEs in scientific and engineering contexts.