This edition of the Reader’s Companion accompanies Skills Matter: Additional Results from the Survey of Adult Skills that reports the results from the 39 countries and regions that participated in the 3 rounds of data collection in the first cycle of PIAAC, with a particular focus on the 6 countries that participated in the third round of the study (Ecuador, Hungary, Kazakhstan, Mexico, Peru and the United States). It describes the design and methodology of the survey and its relationship to other international assessments of young students and adults. The Survey of Adult Skills, a product of the OECD Programme for the International Assessment of Adult Competencies (PIAAC), was designed to provide insights into the availability of some key skills in society and how they are used at work and at home. The first survey of its kind, it directly measures proficiency in several information-processing skills – namely literacy, numeracy and problem solving in technology-rich environments.