This paper discusses the conditions for the grammaticalization of evidentiality in different languages, with special attention to the languages of South Eastern Europe and the Middle East. In these languages, evidentially marked discourse is opposed to neutral discourse, while in some other languages, evidential markers are necessarily included in every verb form; and in others, evidentiality is only lexically expressed. The last section deals with the problem of cross-language comparisons of evidential systems. A few thoughts are presented on the question of how to build a set of concepts to be used as a tertium comparationis.