The following interdependencies and impacts of climate change for existing and newly built office buildings are common knowledge: climate change will negatively impact on thermal comfort of office users by rising indoor temperatures in summer productivity of office workers is directly negatively influenced by elevated indoor temperatures reduced thermal comfort thereby raises work force costs (as salaries make up for the single most important budget point of the majority of enterprises) in order to counteract, it will be necessary to implement mechanical cooling on large scale. Mechanical cooling strongly depends on the availability of electricity at peak hours. Due to significantly increased electricity demand this availability of peak power might generally not be guaranteed everywhere at any time in the decades to come. At the same time, any fossil fuel based generation of the required electricity for cooling involves emissions of climate gases which further induce global warming and further aggravate the above mentioned effects. By simulating thermal conditions in nine existing office buildings in Vienna, Austria, this PhD thesis aimed to investigate: 1. the magnitude of problems arising in this kind of buildings due to climate change 2. possible measures to minimize energy demand through optimization of the buildings envelope optimization inside the buildings Emphasis was placed upon parameters of thermal comfort and energy demand. Effects of increasing cooling demand and CO2 emissions were investigated. As impacts of urban heat islands are assumed to further aggravate the above mentioned climate change consequences, these impacts formed a point of analysis as well. Potential for improvements in the buildings envelope (insulation, quality of windows) and its usage (internal loads, usage profiles, ventilation strategies) were assessed.