The World Health Organization (WHO) 'age-friendly city' framework has been adopted by an increasing number of cities around the world as a key strategy to respond to the needs of their ageing population. Starting with 33 cities in 2006, the membership of the Global Network of Age-Friendly Cities and Communities had grown to 937 cities in 2019, covering a total of 240 million people worldwide. Although the age-friendly movement has been in existence for a decade or more, limited information is available about the development of age-friendly policies. This thesis aims to fill this research gap, exploring how the WHO age-friendly city framework has been interpreted and implemented within local and municipal government. To achieve this goal, the study used a cross-national, multiple case study approach to compare the development of age-friendly policies in three cities: Brussels (Belgium), Manchester (UK) and Montreal (Canada). The policy cycle model was adopted as an analytical lens to guide data collection and data analysis and was used to break the policymaking process into a series of five stages (i.e. agenda setting, policy formulation, decision-making, policy implementation, evaluation). Analysis of each case study was informed by in-depth interviews conducted with 44 stakeholders as well as a review of the key scientific and grey literature on the three age-friendly programmes. The findings of this thesis document how age-friendly policies have been developed in practice and the various processes involved. Contrasting the experience of three cities from three different countries demonstrates that the age-friendly approach can be interpreted differently from one city to another. It demonstrates that depending on their understanding of the age-friendly approach, cities will adopt different aims for their programme, involve different actors in its delivery, and organise their work differently. This thesis suggests that the development of age-friendly policies should be considered as a process, breakable into a series of steps, and that more attention should be paid to the impact of each step on the results of age-friendly policies. Learning from the experience of Brussels, Manchester and Montreal allows a broader reflection on the age-friendly movement and the identification of various challenges facing its development. The thesis concludes by making a series of practical recommendations to address these and to improve the development and delivery of age-friendly policies. By looking at the age friendly movement from a public policy perspective and documenting how cities are working towards increasing their level of age-friendliness, this thesis makes an original contribution to the theoretical and empirical development of the age-friendly movement. As one of the first cross-national comparisons of age-friendly policies, this thesis also contributes to methodological development of this topic and provides pointers for future comparative research on age-friendly cities. The results are likely to be of interest to both researchers and practitioners interested or involved in developing age-friendly work at all levels.