Age-friendly built environments have been promoted by the World Health Organisation (WHO, Geneva, Switzerland) under the Global Age-friendly Cities (AFC) movement in which three domains are related to the built environment. These are: housing, transportation, outdoor spaces and public buildings. The aim is to foster active ageing by optimising opportunities for older adults to maximise their independent living ability and participate in their communities to enhance their quality of life and wellbeing. An age-friendly built environment is inclusive, accessible, respects individual needs and addresses the wide range of capacities across the course of life. Age-friendly housing promotes ageing in familiar surroundings and maintains social connections at the neighbourhood and community levels. Both age-friendly housing and buildings provide barrier-free provisions to minimise the needs for subsequent adaptations. Age-friendly public and outdoor spaces encourage older adults to spend time outside and engage with others against isolation and loneliness. Age-friendly public transport enables older adults to get around and enhances their mobility. For achieving an age-friendly living environment, a holistic approach is required to enable independent living, inclusion and active participation of older adults in society. The eight domains of the AFC movement are not mutually exclusive but overlap and support with one another.
The world population is growing older according to the World Population Prospects 2019 published by the United Nations. Persons aged 65 or above outnumbered children under five years old globally for the first time in history in 2018. One in eleven people in the world (9%) were aged 65 or above in 2019, which is projected to increase to one in six people (16%) by 2050. This figure is even projected to be one to four (25%) in Europe and North America by 2050. Lower birth rates and higher life expectancies are transforming the age pyramid shape, especially for developed countries. Another significant trend is urbanisation, which is projected to increase from 53% of people living in urban areas in 2020 to 70% in 2050 
. In response to the twin factors of population ageing and urbanisation, the World Health Organisation (WHO, Geneva, Switzerland) published Global Age-friendly Cities: A Guide (AFC Guide) in 2007 to enhance the age-friendliness of urban environments 
. The demographic change towards a much older population structure has considerable consequences for the built environment, social welfare and community services. There is a framework of eight domains to enhance the age-friendliness of cities which can be classified into three categories: (1) the built environment to cover housing, outdoor spaces, public buildings and transportation; (2) social aspects to cover respect and social inclusion, employment, social and civic participation; and (3) service provisions to cover community support and health services, communication and information. The aim of this encyclopedic entry is to unfold the underlying idea of the AFC movement, highlight the importance of creating age-friendly built environments and provide recommendations.
The Global AFC movement can be traced back to the resolution 33/52 of the United Nations General Assembly on 14 December 1978 in which worldwide attention was called to the problems of ageing 
. Subsequently, the First World Assembly on Ageing was held in Vienna in 1982, resulting in the Vienna International Plan of Action on Ageing with 62 recommendations for formulating relevant policies and programmes on ageing 
. After 20 years, the Second World Assembly on Ageing was held in Madrid in 2002 to review the implementation of the previous Vienna International Plan and to release the Madrid International Plan of Action on Ageing which focused on three priority areas: older persons and development, advancing health and wellbeing into old age, and ensuring enabling and supportive environments 
As a contribution to the Second World Assembly on Ageing, the WHO published the Active Ageing: A Policy Framework in 2002 to formulate action plans that promote health and active aging 
. The underlying idea of the AFC movement is the policy framework for active ageing. Active ageing aims to extend healthy life expectancy and enhance quality of life as people age. The word ‘active’ does not merely refer to physically active but also refers to the continual participation in social, economic, civil, cultural and spiritual affairs according to personal needs, desires and capabilities. Active ageing arouses people’s awareness of their social, physical and mental wellbeing and highlights the importance of maximising autonomy, mobility and engagement. On one hand, maintaining independence to make personal decisions in relation to daily living based on own preferences is a key goal; on the other hand, maintaining social interaction including intergenerational relationships for older adults is crucial as ageing takes place within the context of others, including friends, neighbours and family members. Since the decline in personal abilities and skills vary from a life course perspective, the supportive and enabling living environments promoted by the age-friendly city movement thus do not only cater for older adults but also address the needs of a diverse group of people.
The WHO Global Network of Age-friendly Cities and Communities (GNAFCC) was then launched in 2010. It is a broad-scale effort under rapid expansion and now involves a total of 1114 cities and communities in 44 countries 
. The Global Network serves as a platform for providing connection and support among different communities and cities worldwide to inspire change and to find appropriate innovative and evidence-based solutions. The vision of the Global Network is for every city and community to strive to become increasingly age-friendly 
For achieving an age-friendly living environment, a holistic approach is required to enable independent living, inclusion and active participation of older adults in society. The eight domains of the AFC Guide are not mutually exclusive but overlap and support with one another. For example, respect and social inclusion are reflected in the barrier-free access to outdoor spaces and public buildings. Availability of communication and information allow people to connect for social and civic participation.