In a compact geodesic metric space of topological dimension one, the minimal length of a loop in a free homotopy class is well-defined, and provides a function l : π 1 (X) −→ R + ∪ ∞ (the value ∞ being assigned to loops which are not freely homotopic to any rectifiable loops). This function is the marked length spectrum. We introduce a subset Conv(X), which is the union of all non-constant minimal loops of finite length. We show that if X is a compact, non-contractible, geodesic space of topological dimension one, then X deformation retracts to Conv(X). Moreover, Conv(X) can be characterized as the minimal subset of X to which X deformation retracts. Let X 1 , X 2 be a pair of compact, non-contractible, geodesic metric spaces of topological dimension one, and set Y i = Conv(X i ). We prove that any isomorphism φ : π 1 (X 1 ) −→ π 1 (X 2 ) satisfying l 2 • φ = l 1 , forces the existence of an isometry Φ : Y 1 −→ Y 2 which induces the map φ on the level of fundamental groups. Thus, for compact, noncontractible, geodesic spaces of topological dimension one, the marked length spectrum completely determines the subset Conv(X) up to isometry. furthermore, the length of the intersection is exactly equal to the length of the original geodesic segment. We then proceed to show that this correspondence is well-defined (i.e. does not depend on the pair of geodesic loops one constructs), and preserves concatenations of geodesic segments. This is used to construct an isometry between the sets of branch points. Using completeness, we extend this to an isometry between the closures of the sets of branch points. Finally, we consider points in Conv(X 1 ) which are not in the closure of the set of branch points. It is easy to see that each of these points lies on a unique maximal geodesic segment, with the property that the only branch points occur at the endpoints of the segment. The correspondance between geodesic segments can be used to see that there is a unique, well-defined, corresponding segment in Conv(X 2 ) of precisely the same length, allowing us to extend our isometry to all of Conv(X 1 ).We conclude this introduction with a few remarks. If the spaces X i are tame (i.e. are semi-locally simply connected), then our Main Theorem can also be deduced from some work of Culler and Morgan [CM87] (see also Alperin and Bass [AB87]). But of course, there are numerous examples of geodesic length spaces of topological dimension one which are not semi-locally simply connected (Hawaiian Earrings, Menger curves, Sierpinski curves, etc.), for which our result does not a priori follow from theirs. Another nice class of examples are Laakso spaces with Hausdorff dimension between one and two [Laa00]. These spaces have nice analytic properties, and work regarding the spectra of the Laplacian has been carried out on them [RS09]. In view of the connections between such spectra and the length spectrum in other contexts, this seems like a most interesting example.A preliminary version of this paper was posted by the second author on the arXiv ...