With the advancement of technology and the information explosion, integration of the two into performance aiding systems can have a significant impact on operational and maintenance environments. The Department of Defense and commercial industry have made great strides in digitizing and automating technical manuals and data to be presented on performance aiding systems. These performance aides are computerized interactive systems that provide procedures on how to operate and maintain fielded systems. The idea is to provide the end-user a system which is compatible with their work environment. The purpose of this paper is to show, historically, the progression of wearable computer aiding systems for maintenance environments, and then highlight the work accomplished in the design and development of glasses- mounted displays (GMD). The paper reviews work performed over the last seven years, then highlights, through review of a usability study, the advances made with GMDs. The use of portable computing systems, such as laptop and notebook, computers, does not necessarily increase the accessibility of the displayed information while accomplishing a given task in a hands-busy, mobile work environment. The use of a GMD increases accessibility of the information by placing it in eye sight of the user without obstructing the surrounding environment. Although the potential utility for this type of display is great, hardware and human integration must be refined. Results from the usability study show the usefulness and usability of the GMD in a mobile, hands-free environment.