During the last years wikis have become important information portals. Nevertheless, research in wiki accessibility is an often neglected topic. This paper takes a basic analytical and systematic step inside this new and complex area, explores the peculiarities, possibilities and limitations of wiki accessibility. Wikis give rise to certain accessibility limitations but also do offer new ways and concepts to improve accessibility. We present basic concepts and extensions to the wiki markup that are to our minds inevitable for improving the accessibility of wikis. It is pointed out, that wiki accessibility is not something totally new but mostly a complex conjunction of yet separated classical discussions. However there are also wiki specific accessibility aspects.