Washington Certificate of Service is a legal document used in Washington State to prove that a particular document has been served. It is also known as a Proof of Service. There are two types of Washington Certificate of Service: Personal Service and Substitute Service. Personal Service requires the document to be delivered to the recipient in person. Substitute Service allows the document to be delivered to an authorized representative, such as a registered agent, or to be mailed in a specific manner. The Washington Certificate of Service must be completed by the person who served the document and must be filed with the court. The certificate must include information about who was served, how it was served, and when it was served.