This report is a review of Human Factors literature a~ d~military specifications concerning Head-up Displays (HUDs). The objectiv ,* is to identify important categories of Human Factors research concerning~virtual-image displays. These research categories are questions that must be answered before specifications can be written for the optimal design of HUDs.The review encompassed an exhaustive list of references available through the Defense Documentation Center (DDC) as well as other pertinent sources not given in the DDC listing.Each requirement in the General Specification for Head-up Displays, MIL-D-81641 (AS) , was compared with the available data. The data base for requirements and the importance of further research concerning each requirement were qualitatively rated. Categories of necessary research were established.Human Factors knowledge has not kept pace with the proliferating uses of HUDs and the expansion of HUD technology. Consequently, the majority of existing Human Factors specifications for HUDs are based on expert opinion rather than empirical data. Several categories of research are required to provide an adequate data base for future specifications, and to understand how specific issues in the design of HUDs affect performance.