Preface.- Linking Systems.- Sandwich Systems.- Linking Sandwich Sets.- The Monotonicity Trick.- Infinite Dimensional Linking.- Differential Equations.- Schr?dinger Equations.- Zero in the Spectrum.- Global Solutions.- Second Order Hamiltonian Systems.- Core Functions.- Custom Monotonicity Methods.- Elliptic Systems.- Flows and Critical Points.- The Semilinear Wave Equation.- Nonlinear Optics.- Radially Symmetric Wave Equations.- Multiple Solutions...