This book provides an up-to-date description of the methods needed to
face the existence of solutions to some nonlinear boundary value
problems. All important and interesting aspects of the theory of
periodic solutions of ordinary differential equations related to the
physical and mathematical question of resonance are treated. The author
has chosen as a model example the periodic problem for a second order
scalar differential equation. In a paedagogical style the author takes
the reader step by step from the basics to the most advanced existence
results in the field.