This chapter critically discusses the most important concepts for a comparative analysis of industrial relation systems: collective bargaining centralisation, bargaining coverage, social partners’ involvement in routine policy-making, and union density. Each dimension will be analysed from a qualitative and quantitative point of view to describe the most significant changes incurred in the last thirty years to bargaining structures in the four models of growth and inequality. The chapter also deals with the effects of bargaining structures on income levels and distribution. The analysis relies on the data collected by Jelle Visser and OECD.