Librational motion in celestial mechanics is generally associated with the existence of stable resonant configurations and signified by the existence of stable periodic solutions and oscillation of critical (resonant) angles. When such an oscillation takes place around a value different than 0 or π, the libration is called asymmetric. In the context of the planar circular restricted three-body problem (CRTBP), asymmetric librations have been identified for the exterior meanmotion resonances (MMRs) 1 : 2, 1 : 3 etc. as well as for co-orbital motion (1 : 1). In exterior MMRs the massless body is the outer one. In this paper, we study asymmetric librations in the 3-dimensional space. We employ the computational approach of Markellos (1978) and compute families of asymmetric periodic orbits and their stability. Stable, asymmetric periodic orbits are surrounded in phase space by domains of initial conditions which correspond to stable evolution and librating resonant angles. Our computations were focused on the spatial circular restricted three-body model of the Sun-Neptune-TNO system (TNO = trans-Neptunian object). We compare our results with numerical integrations of observed TNOs, which reveal that some of them perform 1 : 2-resonant, inclined asymmetric librations. For the stable 1 : 2 TNOs librators, we find that their libration seems to be related with the vertically stable planar asymmetric orbits of our model, rather than the 3-dimensional ones found in the present study.keywords circular restricted TBP -exterior resonances -spatial asymmetric periodic orbits -trans-Neptunian object dynamics ) used an analytical approach to show the existence of asymmetric librations in 1 : p exterior MMRs and their absence in other exterior MMRs as e.g. the 2 : 3 and 3 : 4. We should remark that i) a.p.o are found also in the elliptic planar restricted and in the general planetary planar problem (Antoniadou et al., 2011) ii) 1 : 1 families of a.p.o. also exist and emanate from L 4 for the circular planar restricted (Zagouras et al., 1996) and these families continue in the general planetary problem (Giuppone et al., 2010;Hadjidemetriou and Voyatzis, 2011).Concerning spatial three-body models, families of symmetric periodic orbits (s.p.o.) have been mainly computed for asteroids and TNOs (Ichtiaroglou et al., 1989;Hadjidemetriou, 1993; or for exoplanetary systems (Antoniadou and Voyatzis, 2013;Antoniadou and Voyatzis, 2014). Generally, families of s.p.o. bifurcate from planar periodic orbits which are critical with respect to their vertical stability (Hénon, 1973) and are called vertical critical orbits (v.c.o.). These families extend up to some critical value of the inclination, i, or terminate at i = 180 • (planar retrograde orbit). With respect to their linear stability, the studies cited above showed that most prograde orbits of moderate or high inclination are unstable. Nevertheless, segments of stable orbits also exist providing restricted phase space domains, where resonant angles librate around 0 or π. Markellos (1978)...