  Nearly five decades have elapsed since the seminal 1966 paper of P.W.
Anderson on the flow of superfluid helium, $^4$He at that time. Some of his
"Considerations" -- the role of the quantum phase as a dynamical variable, the
interplay between the motion of quantised vortices and potential superflow, its
incidence on dissipation in the superfluid and the appearance of critical
velocities, the quest for the hydrodynamic analogues of the Josephson effects
in helium -- and the way they have evolved over the past half-century are
recounted below. But it is due to key advances on the experimental front that
phase slippage could be harnessed in the laboratory, leading to a deeper
understanding of superflow, vortex nucleation, the various intrinsic and
extrinsic dissipation mechanisms in superfluids, macroscopic quantum effects
and the superfluid analogue of both {\it ac} and {\it dc} Josephson effects --
pivotal concepts in superfluid physics -- have been performed. Some of the
experiments that have shed light on the more intimate effect of quantum
mechanics on the hydrodynamics of the dense heliums are surveyed, including the
nucleation of quantised vortices both by Arrhenius processes and by macroscopic
quantum tunnelling, the setting up of vortex mills, and superfluid
interferometry.
