What do I do if I don't know the answer to a question in a job interview?

Be honest — but structured. The worst response is silence or an obviously fabricated answer. Better: 'I don't know that with certainty right now. What I do know is [related knowledge]. I would like to research this and provide you with a well-founded answer afterwards.' ISO 10667-1 on interview validity: Competent not-knowing is a quality signal — it demonstrates epistemic discipline and self-assessment ability. OPM: Structured interviews explicitly evaluate self-reflection. What never helps: Bluffing and hoping it goes unnoticed — experienced interviewers spot it immediately. After the interview: Research the open question and briefly address it in your follow-up email — this shows initiative.