This book contains the Proceedings of the 5th International Conference on Improving Energy Efficiency in Commercial Buildings - IEECB'08 which was held in Frankfurt, Germany, 10 - 11 April 2008. The IEECB'08 conference has been very successful in attracting a large international audience, representing a wide variety of stakeholders involved in policy implementation and development, research and programme implementation, investments and property management of energy efficient commercial buildings. IEECB'08 has provided a unique forum to discuss and debate the latest developments in energy and environmental impact of commercial buildings and the installed equipment and lighting.