  We analyse two issues that arise in the context of (matrix) string theories
in plane wave backgrounds, namely (1) the use of Brinkmann- versus
Rosen-variables in the quantum theory for general plane waves (which we settle
conclusively in favour of Brinkmann variables), and (2) the regularisation of
the quantum dynamics for a certain class of singular plane waves (discussing
the benefits and limitations of regularisations of the plane-wave metric
itself).
