Head-Mounted Display (HMD) interfaces are designed to present virtual information within users' real-world view, enhancing their perception of the real world and supporting effective task execution. Effective human performance with an HMD depends strongly on the design of its interface. Three key components are visual depth, clutter, and transparency. HMD interface visual depth is the distance between the user’s eyes and the virtual information displayed in the real-world view. HMD interface clutter denotes the amount of virtual information shown. HMD interface transparency aims to balance the legibility of virtual elements and the visibility of the real-world environment seen through the HMD. This talk presents some of our recent findings on these issues, with an emphasis on human performance and perception.