Topological insulators are materials that insulate in their bulk, but
conduct electric current on their surface. The conducting states on the
surface are, to some extent, protected by topological properties of the
electronic band structure. I will discuss the extent of this protection,
focusing on the effect of disorder on insulators known as "weak
The talk will be mostly based on works carried out with Yuval Baum, Kobi
Kraus and Zohar Ringel. It will NOT assume prior familiarity with the
field of topological insulators.