In the light of social and economic challenges posed by rapid population ageing there is an increased need to understand ageism – how it is expressed and experienced, its consequences and the circumstances that contribute to more or less negative attitudes to age. Ageism is the most prevalent form of discrimination in the UK (Abrams et al., 2011a), estimated to cost the economy £31 billion per year (Citizens Advice, 2007). It restricts employment opportunities, and reduces workplace productivity and innovation (Swift et al., 2013). Ageism also results in inequality and social exclusion, reducing social cohesion and well-being (Abrams and Swift, 2012; Stuckelberger et al., 2012; Swift et al., 2012). Not only is ageism a barrier to the inclusion and full participation of older people in society, but it also affects everyone by obscuring our understanding of the ageing process. Moreover, by reinforcing negative stereotypes, ageism can even shape patterns of behaviour that are potentially detrimental to people's self-interest (Lamont et al., 2015). Here we review national and some international research from the last 25 years to reveal what our core attitudes to ageing are and how they result in discrimination and other damaging consequences. We outline the prevalence of perceived age-based discrimination and its consequences for individuals and society, and then explore the individual and societal factors that contribute to more positive or negative attitudes to age and their application to reducing experiences of ageism. We conclude by considering areas that are likely to be key for policy, research and practice.