Assistance in the collection of taxes enables mutual enforcement and conservancy under domestic law, subject to public policy safeguards. Article 26A establishes mutual assistance for collection of revenue claims, defined to include taxes, interest, penalties and collection costs, provided taxation is consistent with the Convention. A requested State may collect or take conservancy measures on behalf of the requesting State under its own laws as if the claim were its own. Accepted claims gain no special time limits or priority in the requested State. Disputes on existence, validity or amount are for the requesting State's courts. Assistance is subject to domestic law, public policy and proportionality safeguards, and requires prompt notification if claim conditions change.