This book is mainly concerned with the bifurcation theory of ODEs. Chapters 1 and 2 of the book introduce two systematic methods of simplifying equations: center manifold theory and normal form theory, by which one may reduce the dimension of equations and change forms of equations to be as simple as possible. Chapters 3-5 of the book study in considerable detail the bifurcation of those one or two dimensional equations with one, two or several parameters.