This paper argues that the growth of agency work is due to an increase in the replaceability rate of the core workforce. It first re-conceptualises the distinction between core and periphery as deriving both from skill and production requirements (Doeringer and Piore 1971) and from Internal Labour Market arrangements – e.g. qualification requirements and career ladders – bargained at workplace (Rubery 1978). So standardisation and fragmentation of work as well as power shifts between labour and management lead to re-define (and to blur) the boundaries between core and peripheral jobs. The preliminary findings point out changes in the structure of the labour process (e.g. routinization and otusourcing) as well as neo-liberal labour market reforms as responsible for higher replaceability. The findings rely on the data on skills and tasks of the German Federal Institute of Vocational Training and on qualitative interviews with HR managers and workers’ representatives in German automotive companies.