Abstract. We consider a beam whose cross section is a tubular neighborhood, with thickness scaling with a parameter δε, of a simple curve γ whose length scales with ε. To model a thin-walled beam we assume that δε goes to zero faster than ε, and we measure the rate of convergence by a slenderness parameter s which is the ratio between ε 2 and δε. In this Part I of the work we focus on the case where the curve is open. Under the assumption that the beam has a linearly elastic behavior, for s ∈ {0, 1} we derive two one-dimensional Γ-limit problems by letting ε go to zero. The limit models are obtained for a fully anisotropic and inhomogeneous material, thus making the theory applicable for composite thin-walled beams. The approach recovers in a systematic way, and gives account of, many features of the beam models in the theory of Vlasov. 1. Introduction. Composite, i.e., anisotropic and inhomogeneous, thin-walled beams have been extensively studied by the engineering community. We refrain from citing the abundant literature on the subject; we quote, instead, the opening lines of the abstract of [12]: "There is no lack of composite beam theories. Quite to the contrary, there might be too many of them. Different approaches, notation, etc., are used by authors of those theories, so it is not always straightforward to compare the assumptions made and to assess the quantitative consequences of those assumptions." This excerpt well describes the status of the research on composite thin-walled beams. To shed some light on the huge variety of models present in the literature, it is necessary to take a rigorous approach, possibly free of assumptions. The aim of this paper is to derive mechanical models for composite thin-walled beams by Γ-convergence (see [2]), starting from the three-dimensional theory of linear elasticity.This line of research essentially started in [6], where a mechanical model was obtained for an isotropic homogeneous and linearly elastic thin-walled beam with rectangular cross section. In that paper the long side of the rectangle scaled with a small parameter ε > 0 and the other with ε 2 . This "double" scaling was chosen to model a thin-walled beam. One of the main results was a compactness theorem in which different orders of convergence for the various components of the displacement were established. Namely, a sequence of displacements with equi-bounded energy is such that