A first course on differential equations, aimed at engineering students. The prerequisite for the course is the basic calculus sequence. This OER is usable as a standalone textbook or as a companion to a course using another book, such as Edwards and Penney, Differential Equations and Boundary Value Problems: Computing and Modeling or Boyce and DiPrima, Elementary Differential Equations and Boundary Value Problems (section correspondence to these two is given). The author developed and used this book to teach Math 286 and Math 285 at the University of Illinois at Urbana-Champaign. The author also taught Math 20D at the University of California, San Diego with this book. There is enough material to run a two-quarter course, and even a two-semester sequence, depending on lecturer speed.