Form Domains and Eigenfunction Expansions for Differential Equations with Eigenparameter Dependent Boundary Conditions Form domains are characterized for regular $2n$-th order differential equations subject to general self-adjoint boundary conditions depending affinely on the eigenparameter. Corresponding modes of convergence for eigenfunction expansions are studied, including uniform convergence of the first $n-1$ derivatives. Categories:47E05, 34B09, 47B50, 47B25, 34L10