First-Order Linear Differential Equations for Undergraduate Studies is intended to serve as an introductory e-book as well as a main reference for diploma and degree students undertaking ordinary differential equations. The e-book is a collection of lecture notes and outcome of writers' experiences that we have been developed over the years in teaching ordinary differential equations to mathematical and science students. The contents and layout of this e-book have been tailored to cater to the needs of a broad spectrum of new readers. The notes are simple and comprehensive. We hope this e-book will motivate students to enhance their understanding and problem-solving skills as they progress in the course. This e-book consists of three chapters covering major fundamental topics in first-order ordinary differential equations. Chapter 1 introduces the basic terms and terminologies related to differential equations. Chapter 2 discusses several types of first-order ordinary differential equations and the methods of their solutions. The applications of first-order ordinary differential equations are then discussed in Chapter 3. Every chapter is divided into several sub-chapters, and their contents are explained in detail and arranged in a well logical order to make learning easy. Each chapter contains several worked-out examples to assist and guide the students in developing problem-solving skills. They can test and improve their understanding and skills by doing exercises provided at the end of every chapter. We hope that this e-book will be useful and adequate for the students’ needs in learning differential equations, especially those in the field of mathematical sciences and engineering.