Six OECD countries (New Zealand, the United Kingdom, Türkiye, Poland, Lithuania, and Ireland) have introduced auto-enrolment into the third pension pillar. The latest one, implemented in Ireland, begins operating in 2026. All projects are designed to achieve wide coverage of voluntary pension schemes among employees, to supplement moderate replacement rates from the public ones. Using case studies and a multi-dimensional comparative analysis, the study identifies key factors determining the success of these implementations. The research focuses on behavioural design, financial incentives, and the institutional and macroeconomic environment to identify factors that affect participation rates. The collected data show that auto-enrolment schemes constitute an attractive way of saving due to the offered subsidies or tax exemptions and have achieved wide coverage among the population (above 50%, except for Türkiye). Currently, the highest coverage is achieved in the United Kingdom, despite the highest contribution rate. This demonstrates that factors such as the quality of the institutional environment and a culture of saving in occupational schemes are crucial for auto-enrolment success, while a higher contribution burden is not discouraging. Additionally, the findings indicate that the structure of financial incentives affects participation rates across income groups: flat-rate subsidies attract low-income earners and lead to more even coverage, whereas tax exemptions are more favourable to high-income earners, resulting in participation that rises with income.