Historically, numeracy has tended to be forgotten and overlooked in adult education, especially compared to literacy. Yet evidence exists to show that numeracy should be made a priority, and that building the foundational numeracy skills of young people and adults is vital for their well-being in work and life in the 21st century. In the past three decades, there has been an increasing awareness of the role and importance of mathematics and numeracy skills in adult life. This concerns every adult as an individual, as a member of society and as a worker, and how proficiency in these areas is critical in underpinning the skills necessary to negotiate the challenges of 21st-century life. This article describes how this growing understanding and awareness of numeracy has been enhanced through the evolution of the assessment of numeracy in international adult skills surveys. It began with the International Adult Literacy Survey (IALS) in the 1990s, continued with the Adult Literacy and Lifeskills (ALL) survey in the mid-2000s, and in 2011 finally led to the Programme for the International Assessment of Adult Competencies (PIAAC). The latter’s second cycle is due to start collecting data in 2021–2022 and the results are due to be published in 2023. The development and ongoing refinement of the theoretical frameworks and constructs that underpin these programmes and the assessments themselves, alongside the research based on the rich data of empirical and background information emerging from these surveys, have contributed significantly to our knowledge and understanding of numeracy in people’s lives.