This paper discusses theoretical issues linked to the study of identities and intergroup relations within multicultural societies. The issues discussed concern how identities acquire meaning in contemporary societies where events in the social context challenge identities and redefine the boundaries of the social groups. More specifically the processes involved in the formation of new identity categorisations and the structure of identity are discussed. Two arguments are developed. Firstly that within multicultural societies one should study the systems of categorisation and the new psychological boundaries within the wider theories about the world people elaborate and share. Secondly, it is argued that since people's identities reflect social relationships, elements of the structure of common identities might be shared. Finally, the contributions of the papers of this special issue to the above theoretical debate are presented. Copyright © 2000 John Wiley & Sons, Ltd.