This Handbook of Integrated and Sustainable Buildings Equipment and Systems is a direct result of the American Society of Mechanical Engineers (ASME) initiative on Integrated/Sustainable Building Equipment and Systems (ISBES) which has the objective of filling voids in the literature and motivate advances on integrated energy systems in buildings. The main focus of this Volume I of the Handbook is on integrated energy systems and is organized from the current state of knowledge in areas of energy equipment and building energy modeling tools, to emerging topics in a wide range of areas encompassing combined heat and power, building energy storage systems, and advanced control strategies for mechanical energy systems in buildings. In addition, the integration of renewable energy and passive cooling and heating strategies is particularly addressed, closing with advanced techniques to analyze energy demands at the neighborhood and city scales. The contributors have a diverse set of skills and extensive experience in building engineering. The main audience for the Handbook are practitioners building engineers and researchers with the intention of finding current and emerging topics in a single source. This first introductory chapter summarizes the current state of building energy in the global energy sector, and provides a brief synthesis of the strategic areas that the Handbook addresses consistent with the ASME ISBES initiative. The introduction also highlights areas of challenges and opportunities for each of the focus areas, and discusses other topics that may need to be address in the future, and not covered in this Handbook. We hope the readers find the content relevant and useful to their practice and insightful to inspire new advances and developments of Energy Systems for Sustainable Buildings.