The present research thesis explores the way in which modern Augmented and Mixed reality tools are being used in architectural practice and create new hybrid spaces, with which we as users can interact. It is also a research field about the way in which, these spaces can offer the user and the designer new spatial possibilities. Firstly, a clarification and comparison of the concepts included in the broader set of extended reality (XR) is being made and the theoretical and historical context surrounding them is being highlighted. Aiming to explore spaces where the boundaries between “digital” and “real” are eroding, the research then focuses on Augmented and Mixed Reality technologies. Subsequently, the modes of operation and the structure of the above systems are being analyzed. In the main chapter of the research, an analysis of the current applications of AR and MR tools in architecture is being made, through the critical classification and analysis of contemporary examples of case studies. Then, examples of personal case studies, experiments carried out, are presented in the context of investigating the user experience through the use of these tools. By reviewing the the available Greek and foreign literature, as well as personal experimentation, the work aims to clarify the concepts that encompass AR and MR technologies, analyzes  their current usefulness in the individual branches of the architectural and design sector and seeks to answer the research questions that were posed.