Collection assistance in tax treaties requires certified final determination and forwarding of recovered amounts, subject to public policy limits. Article 27 provides that Contracting States will assist each other in collecting taxes, including interest, costs, and civil penalties, upon request by the competent authority certified to be finally determined under that State's law. Collected amounts are forwarded to the requesting authority, and the assisting State may be reimbursed for costs as mutually agreed. The Article does not require a State to adopt collection measures different from those used domestically or contrary to its public policy.