In Chap. 13 we present two functional models for a simple symmetric operator with equal defect numbers. The first of them is given in \(L^2(\Sigma ,{\mathcal H})\) -space (Sect. 13.1), and the second one is given in the reproducing kernel Hilbert space (Sect. 13.2). In Sect. 13.3 these models are used to study additive and singular \({\mathfrak H}_{-1}\) -perturbation \(A_1\) of a selfadjoint operator \(A_0\) in \({\mathfrak H}\) . Generalizations of de Branges–Rovnyak and Carey perturbation results on joint unitary equivalence for the pair \(\{A_0, A_1 \}\) are given by applying the technique of B-generalized boundary triples. For a trace class \({\mathfrak H}_{-1}\) -perturbation \(A_1\) of \(A_0\) the existence of a spectral shift function is proven. If \(A_0\) is a non-negative operator, the abstract version of the Birman-Schwinger principle for a pair of selfadjoint operators \(\{A_0, A_1 \}\) is extended to the case of non-sign-definite perturbations \(A_1\) of \(A_0\) . Proofs of Bargmann’s and Birman-Schwinger’s estimates based on the Kreı̆n resolvent formula are given.