We develop a functional model for operators arising in the study of boundary-value problems of materials science and mathematical physics. We then provide explicit formulae for the resolvents of the associated extensions of symmetric operators in terms of appropriate Dirichletto-Neumann maps, which can be utilised in the analysis of the properties of parameter-dependent problems, including the study of their spectra. §1. Introduction. The need to understand and quantify the behaviour of solutions to problems of mathematical physics has been central in driving the development of theoretical tools for the analysis of boundary-value problems (BVP). On the other hand, the second part of the last century witnessed several substantial advances in the abstract methods of spectral theory in Hilbert spaces, stemming from the groundbreaking achievement of John von Neumann in laying the mathematical foundations of quantum mechanics. Some of these advances have made their way into the broader context of mathematical physics [22,36,44]. In spite of these obvious successes of spectral theory applied to concrete problems, the operator-theoretic understanding of BVP has been lacking. However, in models of short-range interactions, the idea of replacing the original complex system by an explicitly solvable one, with a zero-radius potential (possibly with an internal structure), has proved to be highly valuable [5, 7, 14, 33, 34, 48, 57]. This facilitated an influx of methods of the theory of extensions (both self-adjoint and non-selfadjoint) of symmetric operators to problems of mathematical physics, culminating in the theory of boundary triples.The theory of boundary triples introduced in [23, 25, 31, 32] has been successfully applied to the spectral analysis of BVP for ordinary differential operators and related setups, for example, that of finite "quantum graphs", where the Dirichlet-to-Neumann maps act on finitedimensional "boundary" spaces, see [20] and references therein. However, in its original form this theory is not suited for dealing with BVP for partial differential equations (PDE), see [12, Section 7] for a relevant discussion. The key obstacle to such analysis is the lack of boundary traces 0 u and 1 u for functions u : → R (where is a bounded open set with a smooth boundary) in the domain of the maximal operator A corresponding to the differential expression considered (e.g., the operator − on the domain of L 2 ( )-functions u such that u is in L 2 ( )) entering the Green identity