Across many of the middle- and low-income countries that have participated in the Programme for International Student Assessment (PISA) or are potential new participants, significant proportions of 15-year-olds are not enrolled in school or are in school but in grades below PISA's target sample (Grade 7 and above) and thus not eligible to sit for the PISA test. To help ensure universal coverage of populations in these contexts, in future cycles of PISA and as a contribution to global monitoring of the Sustainable Development Goal (SDG) for education, the OECD is piloting a methodology to include out-of-school youth in the assessment. The pilot is being implemented in Guatemala, Honduras, Panama, Paraguay, Senegal and Zambia. PISA-D's definition of "out-of-school" builds on the work of UNICEF and Unicef Institute for Statistics (UIS) that has defined those children and young people that are excluded from education opportunities. Based on this definition, youth aged 14 to 16 are included in the PISA-D out-of-school sample if they have never attended school, have attended but dropped out during primary school, have completed primary school but did not continue to secondary school, entered secondary school but dropped out, or are currently enrolled in school but are in Grade 6 or below. The sample was expanded from 15-year-olds to 14-16 year-olds on the recommendation of a PISA-D study that highlighted the challenge of locating a single year age group in a household survey in middle- and low-income countries. This brief explains how OECD is establishing methods and approaches to include out-of-school youth in the PISA assessment.