An assignment is the legal transfer of ownership of any property such as a trademark or copyright from one owner to another.  The transferee or "assignee" is the person who acquires ownership, and the transferor or "assignor" is the person who transfers ownership rights.

Great care should be taken when drafting an assignment document and implementing its transfer.  To be valid, a trademark assignment must include the transfer of goodwill accrued in the assigned mark beyond mere assignment of the mark itself. Traditionally, courts have required the transfer of tangible assets along with the trademark.  More recently, however, some courts have taken a more liberal view of assignments, and do not require the transfer of such tangible assets. Nonetheless, the assignee must continue to sell goods or services of the same quality and nature as the assignor to ensure that goodwill has indeed passed with the mark. See PepsiCo, Inc. v. Grapette Co., 416 F.2d 285 (8th Cir. 1969).

You will also want include in the transfer document the assignment of the right to sue for past infringements.  Also consider including a statement about the geographical region for which rights are assigned, the physical transfer of materials establishing the trademark right, and the reasonable cooperation of the assignor, if needed to prove establishment of the trademark.

When a registered trademark is assigned, it is a good idea to record the assignment immediately with the U.S. Patent and Trademark Office, although such recordation is not mandatory.