This paper considers the collapse behaviour of polymeric liners confined within rigid steel pipes. Following a brief survey of two-dimensional models for collapse behaviour, some three-dimensional aspects of the problem are examined. Two particular phenomena are considered: the axial stability of collapse under different loading conditions and the effect of restraint at the liner ends. It is confirmed that the point of onset of unstable collapse is generally well-modelled by the critical pressure predicted from two-dimensional models. The effect of end restraint can generally be neglected, except in the case of short liners, where buckling collapse can be inhibited.