Under trademark law, a trademark owner has the exclusive right to use his/her registered mark. In reality, however, other traders may use or intend to use a trademark that is identical with or similar to such a registered mark. In these circumstances, those who would like to use such a mark may contact the owner of the prior registered trademark to obtain consent, in the form of an agreement, allowing them to use or even to register that similar mark. The purpose of this agreement is to allow the similar trademark to be registered and coexist with the prior registered trademark. It also serves as a guarantee that the use of the similar trademark shall not be deemed as an infringement of the rights of the prior registered trademark owner. This type of agreement often is called a “Trademark Coexistence Agreement” or “Letter of Consent.” Through international comparisons, this article examines the legal status of such agreements in Thailand.