  A set $S$ of vertices of a graph $G$ is a defensive $k$-alliance in $G$ if
every vertex of $S$ has at least $k$ more neighbors inside of $S$ than outside.
This is primarily an expository article surveying the principal known results
on defensive alliances in graph. Its seven sections are: Introduction,
Computational complexity and realizability, Defensive $k$-alliance number,
Boundary defensive $k$-alliances, Defensive alliances in Cartesian product
graphs, Partitioning a graph into defensive $k$-alliances, and Defensive
$k$-alliance free sets.
