The aim of this research is to contribute to the concept of authenticity for the conservation of historic places. The focus of this work is a development of a set of criteria to assess authenticity in order to contribute towards the conservation of historic places in Western Europe.The purpose is to define a set of authenticity criteria. Firstly, this thesis describes the concern of the concept of authenticity in conservation of the built environment and, based on a review of relevant research and theories on how authenticity criteria fit in conservation in general and how it is important to define a set of authenticity criteria in order to conserve historic places in Western Europe. Based on this review of relevant research and theories, the researcher understood the need to define a set of authenticity criteria in order to facilitate the conservation of historic places in Western Europe. The set of criteria to assess authenticity in the conservation of historic places is based on the spirit of UNESCO criteria to test authenticity with the definitions of the criteria used by the USA Parks Service to evaluate integrity of the property in order to be listed. The initial set of criteria is composed of seven qualities as follows: Location, Settings, Design, Workmanship, Materials, Feelings and Association. Linked to the "state of the art " of Conservation of Historic Places in Western Europe, these criteria were applied to four case studies that showed the evidence in all of them, but strongly demonstrates that Location, Feelings and Association are not important criteria for Western Europe. However Function / Use is an important criteria for all Europe. According to the case studies' conclusions and the review of literature, the model of research created is based on five parameters which define the qualities to assess authenticity in conservation. Four of these parameters (Design, Materials, Workmanship and Settings) are based on USA Parks Service definitions.The fifth parameter (Function / Use) is based on the case studies' conclusions. With the support of a sounding board of experts, the author developed a conceptual model for checking the propositions. This model and the propositions were checked by the Delphi Process with a further group of twenty experts from Western Europe and through a continuous analysis of the data. The set of authenticity criteria established was sent to the Delphi panel of experts in Portugal, the UK and West European organisations involved in the conservation of the built heritage with statements in order to find an agreement on the evidence and definition of each criterion. This procedure was based on the assumption that the achievement of a consensus was possible. Consensus was, in fact, obtained about the criteria. The final criteria achieved highlight the emerging importance of function and use in historic places for the future. The final criteria are: Material Design Workmanship Function/Use Setting DEFINITIONS The physical elements that were combined or deposited during a particular period of time and in a particular pattern or configuration to form an historic property. Combination of elements that create the form, plan, space, structure, and style of a property. The physical evidence of the crafts of a particular culture or people during any given period in history or prehistory. The degree of continuity of original or significant uses in a property. The physical environment of a historic property. Due to the fact that the sounding board of experts and Delphi members were made up of people with different backgrounds, ranging from Academia, Architecture, Construction, Industry of Culture, NGOs (Non Governamental Organizations) and other built heritage organizations (UNESCO, ICCROM, ICOMOS, Europa Nostra and Council of Europe) the final set of criteria to assess authenticity for conservation of historic places in Western Europe has an holistic point of view.