This paper presents a theoretical framework for augmenting an existing autopilot by an adaptive element so that it tracks a given smooth reference command with desired specifications. The main contribution of the approach is that it allows for augmenting the autopilot without any modifications to it. The augmentative adaptive element is based on the ℒ1 adaptive output feedback control architecture developed in . The complete path following architecture of this paper enables a UAV with an off-the-shelf autopilot to follow a predetermined path that it was not otherwise designed to follow. The paper concludes with flight test results performed in Camp Roberts, CA, in February of 2007.