  I propose that most problems about circles (cycles, circuits) in ordinary
graphs that have odd or even length find their proper setting in the theory of
signed graphs, where each edge has a sign, $+$ or $-$. Even-circle and
odd-circle problems correspond to questions about positive and negative circles
in signed graphs. (The sign of a circle is the product of its edge signs.) I
outline questions about circles in signed graphs, that seem natural and
potentially important.
