Head mounted displays (HMDs) have disappointed real world users in their inability to live up to over-hyped expectations. This does not, however, mean that HMDs are useless. While still technologically lacking in some areas, appropriately designed HMDs can be extremely useful tools. We will look at the limitations of current HMDs and ways around them. Rather than approach the problem from the optical, electrical and mechanical engineer's point of view, we will approach it from the physiology point of view, answering the question; what is needed to create a useful HMD. The paper is divided into two separate sections. The first, is a description of the performance of the human visual system. The second, addresses how designers attempt to mimic the human visual system in an HMD. This second section will discuss applications that need the specific performance described in section one, current solutions to those needs and finally ideal solutions not yet implemented. Finally, a summary of these findings is presented in a table format.