Collection assistance: treaty parties must assist recovery of finally determined revenue claims under agreed procedural certifications. Article 27 requires Contracting States to assist in collecting treaty-covered taxes and related interest, costs and penalties, subject to a request from the Competent Authority that includes certification the revenue claim is finally determined and an official tax notice. 'Finally determined' means the requesting State has the domestic right to collect and the taxpayer has no further means to restrain collection; the requester must certify exhaustion of recovery efforts. Amounts collected are forwarded to the requesting Competent Authority, with possible reimbursement of costs by mutual agreement, and assistance need not involve measures different from domestic collection practice or contrary to public policy.