The existing building stock in most of the European countries is relatively old and on average 70% of buildings are build before year 1980. The situation in Eastern Europe is even more dramatic and is improving very slowly. Overall the total energy consumption due to building sector is about 40% of all the energy while up to 75% of heating energy is consumed by residential buildings. The major part of the energy is lost in heating the old non-renovated buildings, which consume up to 4 times as much energy as new buildings. This provides an opportunity for large energy savings and has caused rising interest in building retrofi tting. However the retrofi tting process must be done correctly and understanding that these buildings after retrofi tting must serve for next 30 years. As the existing examples show a particular attention must be paid to proper installation of ventilation systems, the correct choice of insulation material and its thickness, as well as proper balancing of heating systems. This handbook will serve as a general guide and inspiration for designers, architects and city planners in choosing the most appropriate retrofi tting solution. The handbook consists of four main chapters and ten annexes. The fi rst chapter explains the terminology used in the handbook. The second chapter introduces the existing building stock situation, climatic conditions and give a general overview of energy supply systems for each of the participating countries. Chapter number three presents the results and describes the retrofi tting processes of case studies as well as familiarizes with applied renewable energy sources. The last chapter gives examples and compares some of the most commonly used simulation programs and calculation tools for building energy performance evaluation in the participating countries. Each participating country in the handbook presents case studies of separate building or building group retrofi tting examples. The chosen cases of Latvia include two kindergartens and one multi-story residential building. Estonia showcases one multi-story residential building in Mustamäe district, one elementary school and one high school. Meanwhile Finland and Sweden present several examples of large-scale renovation of residential areas. Cases of Finland are located in the Helsinki region: one rental housing estate, a residential suburb in the city of Riihimäki, and a mixed residential area in northern Helsinki. The Swedish cases describe the retrofi tting process of three energy effi ciency projects of multidwelling buildings located in Stockholm, Malmö and Umeå. The fi rst case presents a retrofi tting project conducted in 2011 as part of Sustainable Järva, the other two examples include renovation of the residential area Ålidhem in Umeå and Rosengård in Malmö.