A number of metrics for assessing human thermal response to climatic conditions have been proposed in literature over the last decades. They aim at describing human thermal perception of the thermal environment to which an individual or a group of people is exposed. More recently, a new type of "discomfort index" has been proposed in scientific literature, in standards and in guidelines for describing, in a synthetic way, the long-term thermal comfort conditions in buildings and for predicting uncomfortable phenomena. Most of these new indices synthesize the thermal performance of a building in a single value. We present in this paper a systematic review of 15 indices for the long-term evaluation of thermal comfort conditions in a building. They are here grouped in a limited number of homogenous families according to the assumptions on which they are derived and then they are analyzed according to some key aspects in order to identify their scope, their strengths and weaknesses. Finally, some guidance is provided for future developments. (C) 2012 Elsevier B.V. All rights reserved.