Helmet mounted displays are now widely accepted as an essential component of any head coupled night vision system and due to enter service on several rotary wing and fast jet aircraft. The technologies and human factors issues involved in the design and manufacture of HMDs are now reaching maturity. However the operational effectiveness of HMDs is greatly influenced by the design of the HMD system as a whole. This paper presents a discussion of HMD system design issues, covering the design of HMDs for both night and day applications, system architectures and helmet tracker subsystems. The paper then presents an overview of an HMD system which provides both night and day operational capability as part of a head coupled system.