  We generalize three classical characterizations of line graphs to line graphs
of signed and gain graphs: the Krausz's characterization, the Rooij and Wilf's
characterization and the Beineke's characterization. In particular, we present
a list of forbidden gain subgraphs characterizing the class of gain-line
graphs. In the case of a signed graph whose underlying graph is a line graph,
this list consists of exactly four signed graphs. Under the same hypothesis, we
prove that a signed graph is the line graph of a signed graph if and only if
its eigenvalues are either greater than $-2$, or less than $2$, depending on
which particular definition of line graph is adopted.
