The gravitational-wave driven instability of the inertial r-modes in a neutron star may govern the spin-evolution of both nascent and mature accreting neutron stars. In addition the mechanism may generate detectable gravitational waves. However, even though a considerable amount of work has gone into trying to model the r-mode instability serious questions remain. The aim of this talk is to summarise what we think we know and what the most important issues that need to be resolved are. In particular, I will discuss the importance of hyperon bulk viscosity, non-linear inertial mode coupling and superfluid effects.