Assistance in tax collection allows mutual enforcement and conservancy measures under domestic law subject to specified limits. Article 28 obliges Contracting States to assist in collecting revenue claims-taxes, interest, penalties and collection costs-by accepting requests to collect enforceable claims and to apply conservancy measures under the requested State's domestic law as if the claim were its own. Accepted claims receive no special time-limit or priority treatment in the requested State, and disputes over existence, validity or amount must be brought only before the requesting State's courts or administrative bodies. Assistance may be refused where it would conflict with domestic law or practice, public policy, insufficient domestic collection efforts, or when administrative burden is disproportionate.