In-depth interviewing is a promising method. Alas, traditional in-depth interview sample designs prohibit generalizing. Yet, after acknowledging this limitation, in-depth interview studies generalize anyway. Generalization appears unavoidable; thus, sample design must be grounded in plausible ontological and epistemological assumptions that enable generalization. Many in-depth interviewers reject such designs. The paper demonstrates that traditional sampling for in-depth interview studies is indefensible given plausible ontological conditions, and engages the epistemological claims that purportedly justify traditional sampling. The paper finds that the promise of in-depth interviewing will go unrealized unless interviewers adopt ontologically plausible sample designs. Otherwise, in-depth interviewing can only provide existence proofs, at best.Keywords Ontology · Epistemology · In-depth interviewing · Sampling · Probability sampling · Non-probability sampling · Snowball sampling · Purposive sampling · Theoretical sampling What can we learn from in-depth interviewing? Surely, an interviewer can ask multiple respondents myriad questions, but what justifies attending to the answers or analysis? This question motivates the investigation. Because methodological justifications depend crucially on the match between ontological conditions and epistemological assumptions, addressing this question requires assessing the plausibility and coherence of key ontological and epistemological assumptions of in-depth interviewing.Although multiple assumptions ground every method, the study focuses on case-selection for in-depth interview (IDI) studies. Because all researchers must select cases for study, case selection is of broad interest.