  The study of outer disc radius variations in close binary systems is
important for understanding the structure and evolution of accretion discs.
These variations are predicted by models of both quasi steady and
time-dependent discs, and these predictions can be confronted with
observations. We consider theoretical and observational consequences of such
variations in cataclysmic variables and low mass X-ray binaries. We find that
the action of tidal torques, that determine the outer radius at which the disc
is truncated, must be important also well inside the tidal radius. We conclude
that it is doubtful that the tidal-thermal instability is responsible for the
superoutburst/superhump phenomena in dwarf novae, and confirm that it cannot be
the reason for the outbursts of soft X-ray transients. It is likely that tidal
torques play a role during superoutbursts of very-low mass-ratio systems but
they cannot be the main and only cause of superhumps.
