When a boundary layer starts to develop spatially over a flat plate, only disturbances of sufficiently large amplitude survive and trigger turbulence subcritically. After discussing the associated phenomenology, I will show unstable coherent structures (edge states) found numerically, along with the mechanisms responsible for their self-sustenance and their localisation properties. Two examples of boundary layer flows will be treated : the asymptotic boundary layer flow and the Blasius boundary layer. Finally I will show how to incorporate learnings from these unstable solutions in order to build a reduced-order model for the full transition mechanism using a probabilistic cellular automaton.