  The formation and amplification of streamwise velocity perturbations induced
by cross-stream disturbances is ubiquitous in shear flows. This disturbance
growth mechanism, so neatly identified by Ellingsen and Palm in 1975, is a key
process in transition to turbulence and self-sustained turbulence. In this
review, we first present the original derivation and early studies and then
discuss the non-modal growth of streaks, the result of the lift-up process, in
transitional and turbulent shear flows. In the second part, the effects on the
lift-up process of additives in the fluid and of a second phase are discussed
and new results presented with emphasis on particle-laden shear flows. For all
cases considered, we see the lift-up process to be a very robust process,
always present as a first step in subcritical transition.
