Differential equations play an important role in many disciplines including engineering, physics, economics, biology, chemistry etc. A differential equation is an equation that relates derivatives of an unknown function and the function itself. In applications, the unknown function generally represents physical quantities, the derivatives represent their rates of change, and the differential equation defines a relationship between them. The book is for students with intermediate level preparation in mathematics, so exposition is short and clear with detailed solution plans and examples. Minor questions and proofs are omitted. We consider ordinary differential equations only. There are two appendices in the book: complex numbers and polynomials.