Building energy performance assessment is crucial to ascertain the efficiency of energy use in buildings and is the basis to make any decision for enhancing energy efficiency. In order to assess the energy performance of existing buildings quantitatively, the energy use of the assessed buildings should be quantified first. The quantified energy use will be then used to compare with the assessment criteria to determine the energy performance quantitatively. This paper presents an overall review on the state of the art of the research and applications of quantitative energy performance assessment. A framework is proposed for categorizing the energy quantification methods and performance benchmarking methods for energy performance assessment for existing buildings. Energy quantification methods are classified into three categories, i.e. the calculation-based, measurement-based and hybrid methods, according to the energy data acquisition approaches. Energy performance assessment methods are classified according to the assessment scope and depth of assessment, i.e. whole-building benchmarking method at building level and multi-level assessment method. (C) 2012 Elsevier B.V. All rights reserved.