The basic equations of item response theory provide a foundation for inferring examinees' abilities and items' operating characteristics from observed responses. In practice, though, examinees will usually not have provided a response to every available item--for reasons that may or may not have been intended by the test administrator, and that may or may not be related to examinee ability. The mechanisms that produce missingness must be taken into account if correct inferences are to be drawn. Using concepts introduced by D. B. Rubin (1976), the implications for ability and item parameter estimation that are entailed by alternate test forms, targeted testing, adaptive testing, time limits, and omitted responses are discussed. (Contains 2 tables and 17 references.) (Author/SLD)