This article describes a method for discriminating among registers of Modern Greek and among authors within a given register. Two issues have been investigated: (a) whether register discrimination can successfully exploit linguistic information reflecting the evolution of a language (such as diglossia features of the Modern Greek language) and (b) what kind of linguistic information and which statistical techniques may be applied to author discrimination within one register. Using clustering techniques and variables reflecting the diglossia situation, we have successfully discriminated registers in Modern Greek. However, diglossia information on its own has not been shown sufficient for author discrimination within one register. Instead, other linguistic features, including PoS distribution and discourse tendencies, have been combined with methods such as discriminant analysis in order to obtain a high degree of accuracy.