Abstract. Interface controls are unknown functions used as Dirichlet or Robin boundary data on the interfaces of an overlapping decomposition designed for solving second order elliptic boundary value problems. The controls are computed through an optimal control problem with either distributed or interface observation. Numerical results show that, when interface observation is considered, the resulting interface control domain decomposition method is robust with respect to coefficients variations; it can exploit nonconforming meshes and provides optimal convergence with respect to the discretization parameters; finally it can be easily used to face heterogeneous advectionadvection-diffusion couplings.Key words. domain decomposition, optimal control, elliptic boundary value problems, nonconforming discretizations, heterogeneous coupling, ICDD AMS subject classifications. 65N55, 49J20, 49K20, 65N30, 65N35 DOI. 10.1137/120890764 1. Introduction. In this paper we propose an overlapping domain decomposition method with interface conditions that are suitable to face both homogenenous and heterogeneous couplings, i.e., with either the same or different operators in the subdomains.We start by considering a family of overlapping domain decomposition methods, originally proposed in [13] and [16], named least square conjugate gradient (LSCG) and virtual control (VC) methods, respectively, designed for second order self-adjoint elliptic problems. These methods reformulate the overlapping multidomain problem as an optimal control problem whose controls are the unknown traces (or fluxes) of the state solutions at the subdomain boundaries. (For this reason we speak about interface controls.) The constraints of the minimization problem are the state equations, the observation is distributed on the overlap, and the controls are found by either minimizing the L 2 or H 1 norm of the jump between state solutions associated with the subdomains that share the same overlap. When the optimal control problem is solved through the optimality system, by following the classical theory of Lions ([15]), we need to solve both the primal and the dual state problems, as well as to compute the jump between the two solutions on the whole overlap. When non-self-adjoint problems are considered, the matrices associated with the discretization of both primal and dual problems have to be built and stored; moreover the same computational grid on the overlap has to be used, in order to avoid heavy interpolation processes from one grid to another. As we will show in [6], the convergence rate of these methods