  The purpose of this contribution is to summarize and discuss recent advances
regarding the onset of turbulence in shear flows. The absence of a clear cut
instability mechanism, the spatio-temporal intermittent character and extremely
long lived transients are some of the major difficulties encountered in these
flows and have hindered progress towards understanding the transition process.
We will show for the case of pipe flow that concepts from nonlinear dynamics
and statistical physics can help to explain the onset of turbulence. In
particular the turbulent structures ('puffs') observed close to onset are
spatially localized chaotic transients and their lifetimes increase super
exponentially with Reynolds number. At the same time fluctuations of individual
turbulent puffs can (although very rarely) lead to the nucleation of new puffs.
The competition between these two stochastic processes gives rise to a
non-equilibrium phase transition where turbulence changes from a
super-transient to a sustained state.
