Abstract. The concept of weak stability boundary has been successfully used in the design of several fuel efficient space missions. In this paper we give a rigorous definition of the weak stability boundary in the context of the planar circular restricted three-body problem, and we provide a geometric argument for the fact that, for some energy range, the points in the weak stability boundary of the small primary are the points with zero radial velocity that lie on the stable manifolds of the Lyapunov orbits about the libration points L 1 and L 2 , provided that these manifolds satisfy some topological conditions. The geometric method is based on the property of the invariant manifolds of Lyapunov orbits being separatrices of the energy manifold. We support our geometric argument with numerical experiments.Key words. Weak Stability Boundary, Lyapunov Orbits, Invariant Manifolds AMS subject classifications. 70F15, 70F07, 37D101. Introduction. The notion of weak stability boundary (WSB) was first introduced heuristically by Belbruno in 1987 for designing fuel efficient space missions and was proven to be useful in applications [2,3,4,12,13,6,45,33,14,35,22,7,39,38,44]. The WSB can be used to construct low energy transfers to the Moon, requiring little or no fuel for capture into lunar orbit. The first application for an operational spacecraft occurred in 1991 with the rescue of the Japanese mission Hiten. WSB was also applied in ESA's spacecraft SMART-1 in 2004 (see [40]). The WSB technique will be applied again in ESA's mission BepiColombo to explore planet Mercury in 2013 (see [25]), and in some upcoming NASA missions.A different methodology of designing fuel efficient trajectories, based on hyperbolic invariant manifolds, was proposed in [1,26,28,29,30,31], and was successfully applied in several space missions (see also [18,19,20,21]). In some of these works it has been suggested that the hyperbolic invariant manifold method can be used to explain the trajectories obtained through the WSB method. Supporting this assertion, García and Gómez present in [22] numerical explorations that suggests that, for some range of energies, the WSB is contained in the closure of the union of the stable manifolds of the periodic orbits about two of the equilibrium points of the planar circular restricted three-body problem.In this paper we use the separatrix property of the invariant manifolds of the periodic orbits about the equilibrium points to argue that, under some conditions, the points on the stable manifolds are WSB points. We support our geometrical argument with numerical experiments. Our result, corroborated with the result in [22], demonstrates, by double inclusion, that, for some range of energies, the WSB coincides with the set of points on the stable manifolds with zero radial velocity and negative Kepler energy relative to the small primary.In Section 2 we give some background on the planar circular restricted three-body problem. In Section 3 we define the WSB as follows: in the context of the planar circular r...