This volume of selected papers from the 20th International Conference on Historical Linguistics (Osaka, Japan, July 2011) presents a set of stimulating and ground-breaking studies on a wide range of languages and language families. As the scope of studies that can be characterized as 'Historical Linguistics' has expanded, ICHL conferences have likewise seen a broadening of topics presented, and this conference was no exception, reflected by the inclusion in this volume of a plenary presentation on the grammaticalization of expressions of negation and gendered kinship in American Sign Language. Three other papers propose new views of the role of grammaticalization in English, Chinese, and Niger-Congo languages. Four of the papers discuss specific problems that arise in the comparison and reconstruction of linguistic features in a range of languages from Asia, Europe and South America. The last six studies deal with innovative approaches to the historical development of suppletion in Romance languages, possessive classifiers in Austronesian, universal quantifiers in Germanic, adjectival sequences in English, exaptation in Celtic and Early English, and drift in Ancient Egyptian.