  Much evidence has been collected to date which shows that repulsive
electron-electron interaction can lead to the formation of particle pairs in a
one-dimensional random energy landscape. The localization length \lambda_2 of
these pair states is finite, but larger than the localization length \lambda_1
of the individual particles. After a short review of previous work, we present
numerical evidence for this effect based on an analysis of the interaction
matrix elements and an application of the decimation method. The results based
on the decimation method for a two-dimensional disordered medium support the
localization-delocalization transition of pair states predicted recently.
