  A biased graph consists of a graph $G$ together with a collection of
distinguished cycles of $G$, called balanced cycles, with the property that no
theta subgraph contains exactly two balanced cycles. Perhaps the most natural
biased graphs on $G$ arise from orienting $G$ and then labelling the edges of
$G$ with elements of a group $\Gamma$. In this case, we may define a biased
graph by declaring a cycle to be balanced if the product of the labels on its
edges is the identity, with the convention that we take the inverse value for
an edge traversed backwards. Our first result gives a natural topological
characterisation of biased graphs arising from group-labellings.
  In the second part of this article, we use this theorem to construct some
exceptional biased graphs. Notably, we prove that for every $m \ge 3$ and
$\ell$ there exists a minor minimal not group labellable biased graph on $m$
vertices where every pair of vertices is joined by at least $\ell$ edges.
Finally, we show that these results extend to give infinite families of
excluded minors for certain families of frame and lift matroids.
