  This is, mostly, a survey of results about the birational geometry of
rationally connected manifolds, using rational curves analogous to lines in
${\mathbb P}^n$ ({\it quasi-lines}). Various characterizations of a Zariski
neighbourhood of a line in ${\mathbb P}^n$ are obtained, some of them being
new. Also, methods of formal geometry are applied for deducing results of
birational nature.
