The paper contains a simplified and improved version of the results obtained by the authors earlier. Wave propagation is discussed in a network of branched thin wave guides when the thickness vanishes and the wave guides shrink to a one dimensional graph. It is shown that asymptotically one can describe the propagating waves, the spectrum and the resolvent in terms of solutions of ordinary differential equations on the limiting graph. The vertices of the graph correspond to junctions of the wave guides. In order to determine the solutions of the ODE on the graph uniquely, one needs to know the gluing conditions (GC) on the vertices of the graph.Unlike other publications on this topic, we consider the situation when the spectral parameter is greater than the threshold, i.e., the propagation of waves is possible in cylindrical parts of the network. We show that the GC in this case can be expressed in terms of the scattering matrices related to individual junctions. The results are extended to the values of the spectral parameter below the threshold and around it. * The authors were supported partially by the NSF grant DMS-0706928.i.e. Ω ε is the ε-contraction of Ω = Ω 1 .For the sake of simplicity, we shall assume that Ω ε is self-similar in a neighborhood of each junction. Namely, let J j(v),ε be the junction which corresponds to a vertex v ∈ V of the limiting graph Γ. Consider a junction J v,ε = J j(v),ε and all the channels adjacent to J v,ε . If some of these channels have finite length, we extend them to infinity. We assume that, for each v ∈ V, the resulting domain Ω v,ε , which consists of the junction J v,ε and the semi-infinite channels emanating from it, is a spider domain. We also assume here that all the channels C j,ε have the same cross-section ω ε . This assumption is needed only to make the results more transparent (The general case is studied in [22]). From the self-similarity assumption it follows that ω ε is an ε−homothety of a bounded domain ω ⊂ R d−1 .Let λ 0 < λ 1 ≤ λ 2 ... be eigenvalues of the negative Laplacian −∆ d−1 in ω with the BC B 0 u = 0 on ∂ω where B 0 coincides with the boundary operator B on the channels, see (1), with ε = 1 in the case of the third boundary condition. Let {ϕ n (y)}, y ∈ ω ∈ R d−1 , be the set of corresponding orthonormal eigenfunctions. Then λ n are eigenvalues of −ε 2 ∆ d−1 in ω ε and {ε −d/2 ϕ n (y/ε)} are the corresponding eigenfunctions. In the presence of infinite channels, the spectrum of the operator H ε consists of an absolutely continuous component which coincides with the semi-bounded interval [λ 0 , ∞) and a discrete set of eigenvalues. The eigenvalues can be located below λ 0 and can be embedded into the absolutely continuous spectrum. We will call the point λ = λ 0 the threshold since it is the bottom of the absolutely continuous spectrum or (and) the first point of accumulation of the eigenvalues as ε → 0. Let us consider two of the simplest examples: the Dirichlet problem in a half infinite cylinder and in a bounded cylinder of length l. In the fir...