The aim of this case study was to explore the abilities and limitations of trade unions in their response to undeclared work, which has received scant attention in research on working conditions and industrial relations. The authors use power resource theory to examine the outcome of a Swedish government initiative aimed to boost the ability of the social partners to tackle undeclared work. The findings confirm previous literature suggesting cross-sectoral differences in the extent and nature of undeclared work and an association between low levels of power resources and high risk of undeclared work. The authors recommend that future initiatives take account of cross-sectoral differences in the nature and extent of undeclared work and available power resources. Future research should consider how different actors can contribute to the ability of the social partners in different sectors to engage in the battle against undeclared work