Sample undercoverage issues in the National Education Longitudinal Study of 1988 (NELS:88) are addressed. The main focus is the exclusion of certain categories of student in the base year, 1988, and in in-school follow-up rounds. A subsidiary focus is the question of how adequately transfer students were captured within the sampling procedures of the study. Recommendations are offered for how better to deal with undercoverage issues in future school-based longitudinal studies. The six ways in which a student might not have been selected were: (1) refusal by the school to participate; (2) ineligibility of the school; (3) ineligibility of the student, for language, disability, behavioral problems, or lack of English; (4) absence from the school due to study elsewhere; (5) temporary unavailability due to illness or transition; (6) clerical error; and (7) inadequate sampling frame that omitted a school. The exclusion of students is referred to as a problem, but including everyone would have been more of a problem. Ways to increase the rate of meaningful participation in the future are discussed. The experience of NELS:88 suggests that more students have been excluded than is justified. Two tables provide study data. (Contains 28 references.) (SLD)