Using Leray-Schauder degree theory we obtain various existence and multiplicity results for nonlinear boundary value problems φ(u ) = f (t, u, u ), l(u, u ) = 0 where l(u, u ) = 0 denotes the Dirichlet, periodic or Neumann boundary conditions on [0, T ], φ : ]−a, a[ → R is an increasing homeomorphism, φ(0) = 0. The Dirichlet problem is always solvable.For Neumann or periodic boundary conditions, we obtain in particular existence conditions for nonlinearities which satisfy some sign conditions, upper and lower solutions theorems, Ambrosetti-Prodi type results. We prove Lazer-Solimini type results for singular nonlinearities and periodic boundary conditions.