The National Education Longitudinal Studies Program (NELS) is a long-term effort that encompasses the educational experience of youth from three decades. This report is based on the National Education Longitudinal Study of 1988 (NELS:88), a component of the NELS series that began as a national probability sample of eighth grade schools and students in 1988. These students have been followed over time at 2-year intervals, with the most recent followup taking place in 1994. The report is one of a series of methodological monographs that deal with technical issues concerning the analysis of NELS:88 data and the sampling and survey methodology of the study. This methodology monograph addresses issues of student sample exclusion in NELS:88, particularly the problem posed by eighth graders who were declared ineligible for the study owing to factors--physical or mental disabilities, or lack of proficiency in the English language--deemed by their schools to pose a barrier to participation. Chapter 1 explains the exclusion of some students from the survey and discusses the reliability and validity issues exclusion poses. Chapter 2 reviews characteristics of base-year ineligibles, and chapter 3 discusses the eligibility status of the excluded 188 eighth graders in 1992. Chapter 4 considers other potential sources of sample undercoverage, and chapter 5 discusses the need for greater inclusiveness and recommendations to minimize sources of undercoverage. Chapter 6 is a 105-item bibliography. Five appendixes contain three supplementary tables, screening for eligibility reassessment, study eligibility criteria, technical notes, and a glossary. (Contains eight tables, four appendix tables, five figures, and one appendix figure.) (SLD)