Interrogatories are written questions, answered in writing and under oath, that one party serves on another to develop facts, pin down contentions, and lock an opponent into a sworn record. This UpLaw guide works through the full life cycle of the device under Federal Rule of Civil Procedure 33: the twenty-five-question ceiling and how courts count discrete subparts, the relevance-and-proportionality scope borrowed from Rule 26(b)(1), the contention interrogatory and the timing fights it invites, the option to answer with business records under Rule 33(d), and the verification that turns an answer into evidence. It maps how the 2015 amendments killed boilerplate and forced objections to be stated with specificity, and it pairs each recurring objection with model language a practitioner can adapt. The article closes with the duty to supplement under Rule 26(e), the meet-and-confer and motion-to-compel machinery of Rule 37, and a strategy discussion that treats interrogatories as both sword and shield. Worked examples run throughout, all clearly hypothetical, so a litigator, a judge, and a non-lawyer party can each follow the reasoning.