What is discovery in law?

The main tools

• Interrogatories — written questions answered in writing under oath; federal practice limits them to 25 without leave.
• Requests for production — documents, contracts, emails, records and electronically stored information.
• Depositions — live sworn questioning, transcribed.
• Requests for admission — asking the other side to admit specific facts, narrowing what has to be proved.
• Physical or mental examinations — available where a party's condition is genuinely in dispute.

The scope, and its limits

Federal Rule 26 allows discovery of any non-privileged matter relevant to a claim or defence and proportional to the needs of the case. Proportionality is what courts use to cut back sprawling electronic demands. Privileged material — attorney-client communications, attorney work product — is withheld, but must be listed on a privilege log.

The duty to preserve

Once litigation is reasonably anticipated, both sides must stop routine deletion and preserve relevant material. A litigation hold notice is the standard mechanism. Destroying evidence after that point is spoliation, and sanctions range from an adverse-inference instruction to dismissal or default judgment.

Criminal cases are different

Criminal discovery is much narrower and largely one-directional. The prosecution must disclose exculpatory evidence under Brady v. Maryland and impeachment material under Giglio, and statutory rules cover statements and reports — but there is no general right to depose prosecution witnesses in most jurisdictions.