This chapter introduces design and principles of head mounted displays (HMDs), as well as their state-of-the-art examples, for augmented reality (AR). Section 2 introduces a brief history of head mounted displays, human vision system, and application examples of see-through HMDs. Section 3 describes designs and principles of HMDs, such as typical configurations of optics, typical display elements, and major categories of HMDs. Section 4 gives typical characteristics of HMDs, such as resolution, field of view, and distortion. Section 5 describes human perceptual and health issues such as depth perception and safety. Finally, Section 6 gives conclusions with future challenges and prospects. For researchers, learners and HMD developers, this chapter is a good start point to learn basics, state of the art technologies, and future research directions of HMDs. For system developers and end-users, this chapter will give a good insight to HMDs to choose a suitable HMD for their purposes.