  A vertex $w$ of a connected graph $G$ strongly resolves two vertices $u,v\in
V(G)$, if there exists some shortest $u-w$ path containing $v$ or some shortest
$v-w$ path containing $u$. A set $S$ of vertices is a strong metric generator
for $G$ if every pair of vertices of $G$ is strongly resolved by some vertex of
$S$. The smallest cardinality of a strong metric generator for $G$ is called
the strong metric dimension of $G$. In this paper we obtain several tight
bounds or closed formulae for the strong metric dimension of the Cartesian sum
of graphs in terms of the strong metric dimension, clique number or twins-free
clique number of its factor graphs.
