The Directive 2008/1/EC, concerning Integrated Pollution Prevention and Control (IPPC), was transposed in Spain by the Law 16/2002, which created a new administrative figure, the Environmental Integrated Authorization (EIA). To give the IEA, the environmental competent authority must carry out an environmental assessment of the IPPC activity (EA-IPPC) which should take into account all factors involved in any way with the activity, from the design phase to implementation and subsequent operation. The result of this Environmental Assessment will be reflected in a resolution, containing all the constraints that the activity must comply for its exploitation. Due to the high complexity of integrating all the values promoted by the IPPC Directive, and that the current methodology used by the Administration is unspecified, there is the motivation for developing this PhD thesis, with the aim of proposing a new methodology for the EA-IPPC , to which shall be named as Assessment of Integrated Environmental Authorization (AIEA). The AIEA is an application of the concept of multi-criterion procedural rationality. Uncertainties, data gaps and cognitive limitations in the decision-making process, are all peculiarities typical of environmental decision making. In this sense, the AIEA looks beyond predicting the environmental consequences of the decision and ensures that a set of decision criteria, ensuring that it is taking into account the substantive values IPPC, is effectively implemented in the process of making decisions. The evaluation of best available techniques (BAT) has been implemented through multi-criterion decision-making techniques on a real case of IEA. An innovative application of AHP and ANP techniques has been proposed for the assessment of the same BAT, in order to establish a comparative analysis. All these conceptual changes make that the proposal presented in this thesis as a whole, represents a value added for the competent authority, while optimizing its evaluation work, and it is considered as a potential tool for environmental sustainability and for the IPPC activity itself. Moreover, the proposed methodology is an innovation in itself, since there is no previous information in the field of environmental assessment of IPPC projects.