The definition of the study population for a clinical trial via the criteria for trial eligibility has implications for the validity of the study and its applicability to clinical practice. Though issues of equity regarding the selection of subjects for research have long been a concern of ethicists, issues regarding the impact of subject selection on a trial's generalizability have only recently attracted ethical scrutiny. After a review of the history of the ethics of subject selection, I focus on three empirical questions regarding the generalizability of clinical trials. (1) What proportion of diseased populations are studied in clinical trials? (2) How are subjects selected for clinical trial participation (and what are the main barriers to participation)? (3) Are clinical trial participants comparable to non-participants? Finally, the role of the Institutional Review Board--Research Ethics Board in Canada--in assessing the generalizability of clinical research is discussed.