This book presents an up-to-date account of many topics of current interest in the theory of nonlinear ordinary differential equations. They include fixed point theory, periodic problems, lower and upper surfaces, positone and semi-positone problems, singular equations, limit circle problems, finite and infinite interval problems, and impulsive differential equations. In addition many important applications are presented to complement the theory. <br/> <em>Audience:</em> This work will be of interest to theoretical and applied mathematicians, and graduate students in the above disciplines.