The main focus of this thesis is the study of user centered issues in Mixed and Augmented Reality (AR) systems. Mixed Reality (MR) research is in general a highly technically oriented field with few examples of user centered studies. The few studies focusing on user issues found in the field are all based on traditional Human Computer Interaction (HCI) methodologies. Usability methods used in MR/AR research are mainly based on usability methods used for graphical user interfaces, sometimes in combination with usability for Virtual Reality (VR) applications. MR/AR systems and applications differ from standard desktop applications in many ways, but specifically in one crucial respect; it is intended to be used as a mediator or amplifier of human action, often in physical interaction with the surroundings. The differences between MR/AR systems and desktop computer display based systems create a need for a different approach to both development and evaluations of these systems. To understand the potential of MR/AR systems in real world tasks the technology must be tested in real world scenarios. This thesis describes a theoretical basis for approaching the issue of usability evaluations in MR/AR applications. It also includes results from two user studies conducted in a hospital setting where professionals tested an MR/AR system prototype.