The OECD's new Study on Social and Emotional Skills aims to provide policy makers, educators, families and communities with a comprehensive set of tools to foster students' social and emotional learning. The Study's assessment framework -- presented in this paper -- is a result of an extensive literature review of previous research, existing frameworks and assessment approaches in the area of social and emotional skills. The framework, developed by a team of experts in various domains, incorporates evidence from psychology, education, organisational sciences, sociology, economy, and public policy. This framework describes the objectives, characteristics and expected outcomes of the Study. It presents the conceptual model of social and emotional skills assessed in the Study, their development, malleability and predictive value. The framework also discusses how factors in students' family, school and peer environment influence their social and emotional skills' development along with the contextual questionnaires designed to gather this information. The framework also presents the Study's design, assessment approach, instrument development process, sampling procedures and data collection methods.