Causality, linking and refocusing spacetimes V.Chernov (based on a joint work with Yuli Rudyak) We define the invariant $alk$ that generalizes the linking number to the case of nonzero homologous submanifolds and apply it to the study of causality in globally hyperbolic spacetimes $(X, g)$. The space $N$ of null geodesics in $(X, g)$ is identified with the spherical cotangent bundle $ST^*M$ of a Cauchy surface $M$. All the null geodesics passing through $x\in X$ form a sky $S_x\subset N=ST^*M$ of $x.$ As it was observed by Low, if the link $(S_x, S_y)$ is nontrivial, then $x,y\in X$ are causally related. We give many examples of spacetimes for which $alk(S_x, S_y)$\neq 0$ if and only if $x, y\in X$ are causally related. For nonrefocussing $(X, g)$ we show that two events are causally unrelated iff the link of their skies can be deformed to a pair of $S^{m-1}$-fibers of $ST^*M\to M$ by an isotopy through skies. Low proved that if $(X, g)$ is refocusing, then $M$ is a closed manifolds. We prove that the universal cover of $M$ also is a closed manifold and hence $\pi_1(M)$ is finite. We discuss relations between refocusing spacetimes and $Y^l_m$ Riemannian manifolds studied by Berard Bergeri et al.