Where multiple parties face a common adversary, their counsel often wish to share analysis and materials. A common interest or joint defense agreement is the mechanism for doing so without waiving privilege.

It is not a separate privilege. It is an exception to waiver: communications that are already privileged retain protection when shared in furtherance of a common legal interest. Sharing non-privileged material gains nothing, and sharing with a party that has no genuine common legal interest gains nothing either.

Common legal interest, not merely a common commercial one. Jurisdictions differ on whether pending litigation is required.

In writing. An oral arrangement is recognized in many places but leaves the scope contested. A written agreement should state the common interest, what may be shared, that sharing does not waive, that no attorney-client relationship arises with the other parties, and how withdrawal works.

The withdrawal problem. If one party later cooperates against the others, what happens to shared material? Agreements should provide that shared information remains protected and that a departing party may not use it — though enforcement against a government demand is uncertain.

Individual clients require caution. A joint defense with a company can constrain an employee’s later flexibility, and counsel must explain that before joining rather than after.