This book gathers the peer-reviewed papers presented at the 4th International Conference on Protection of Historical Constructions (PROHITECH), held in Athens, Greece, on October 25-27, 2021. The conference topics encompass structural and earthquake engineering, intervention strategies, materials and technologies, digital documentation, architecture and urban planning, cultural heritage, all of which represented by a showcase of case studies covering different construction materials, as well as sustainability, energy efficiency, and adaptation to climate changes. As such the book represents an invaluable, up-to-the-minute tool, providing an essential overview of protection of historical constructions, and offers an important platform to researchers, engineers and architects.