The paper presents a design methodology and a case study for monocular head-mounted displays (HMDs), wherein a user can simultaneously and easily see the physical world by looking through and/or around the display. The design approach is user-focused because of the complexity of the human visual system, and because HMDs are very task, user, and context specific. A literature review of factors related to HMD design is given. This includes considerations for basic optical design, the human visual system, and head and neck biomechanics. General HMD design guidelines are given based on these considerations. For the specific case study on fire-fighting, it is recommended that the HMD be mounted at 15 degrees to 45 degrees below the Frankfurt plane, with a 15 degrees to 40 degrees field of view. A resolution of 20-60 px/deg should be focused at 1 m or farther. The neck joint torque due to the HMD should not exceed about 1 Nm. This equates to a typical maximum weight of 0.5 to 1 kg depending on the mounting location.