The 2023 Survey of Adult Skills, a product of the OECD Programme for the International Assessment of Adult Competencies (PIAAC), provides a comprehensive overview of adults' literacy, numeracy and adaptive problem-solving skills -- skills that are fundamental for personal, economic and societal development. These key information processing skills provide the foundation for access to employment, higher wages and continuous learning, while enabling individuals to navigate the complexities of their personal and civic lives. The Survey of Adult Skills 2023 Technical Report provides a detailed account of the methodology underpinning the survey. It describes the assessment and the sampling design of the survey and outlines the process of developing the cognitive skill measures and the questionnaires used to collect background information on respondents. It explains the approach to scaling the cognitive data and creating the proficiency scales for reporting results. It presents tools for analysing the data and details all operational aspects of the implementation of the survey, including field operations, quality control and data management. In addition, the report explains how the results of the 2023 Survey are linked to those of the first survey cycle, conducted between 2011 and 2018, as well as to previous adult skills surveys.