This chapter analyses the process of labour market dualisation and its consequences for the relation between workers and trade unions in Spain. It examines the effects of the progressive fragmentation of the Spanish work force - i.e. the growing differentiation of a core of workers with stable jobs and a periphery without - on workers’ patterns of collective action, as well as on their attitudes to, and perceptions and evaluation of, trade unions. The chapter argues that the differentiation of unemployment risks by type of contract has produced a two-tier system of employment relations, in which the interests of fixed-term and unemployed workers are largely disregarded. It is important to approach the study of labour market dualisation from a dynamic perspective that stresses the longitudinal character of the phenomena of employment stability and instability. From an employment perspective, two-tier flexibilisation implies that fixed-term contracts, given their low costs, become the main channel of entry into employment.