  We study a number of hierarchical network models related to the
Chalker-Coddington model of quantum percolation. Our aim is to describe the
physics of the quantum Hall transition. The hierarchical network models are
constructed by combining series and parallel composition of quantum resistors.
The localization-delocalization transition occurring in these models is treated
by real space renormalization techniques. Essentially, the
localization-delocalization transition is due to a competition between two
one-dimensional localization mechanisms.
