  We study the turbulence transition of plane Couette flow in large domains
where localised perturbations are observed to generate growing turbulent spots.
Extending previous studies on the boundary between laminar and turbulent
dynamics we determine invariant structures intermediate between laminar and
turbulent flow. In wide but short domains we find states that are localised in
spanwise direction, and in wide and long domains the states are also localised
in downstream direction. These localised states act as critical nuclei for the
transition to turbulence in spatially extended domains.
