This thesis intends to develop a methodology for valuing environmental variables by applying the real options theory. In the attempt to assign a monetary value to environmental variables, several methods have been proposed in the field of economic sciences. However, assigning an economic value to an environmental asset has not been an easy task as it involves a multidisciplinary range of concepts. There are several methods for evaluating environmental resources and among them there are three which appear repeatedly in the literature, such as: Travel Cost Method (TCM), Hedonic Price Method (HPM) and Contingent Valuation Method (CVM). These three techniques are primarily based on consumer preferences, which increase the subjectivity in calculating the respective values. Furthermore, some important aspects involved in the environmental valuation, such as the quasioption value and the existence value, are simply ignored or completely implicit, as in the case of the TCM. Hence, some aspects of the totality of the environmental economic valuation are not taken into consideration. The purpose of this research was to develop a methodology for assessing natural resources using the real options theory approach as well as its inherent concepts. In order to develop a new methodology for assessing natural resources, the problem of valuation has been placed in the context of a case study related to the Brazilian electricity sector. The Brazilian electrical system addresses the hydraulic constraints in a strict way and the violation of these restrictions results in damage to the environment. Due to legal and institutional issues, the hydraulic constraints limit the autonomy of decision regarding the use of water to generate electric power. However, at critical moments, as in periods of drought, it is necessary to contemplate the possibility of making the limits to these restrictions more flexible. This may incur costs that are generally unknown. The violation of these rigorous limits involves a series of aspects that may be of social, economic, political and environmental nature. In order to develop the proposed methodology, this study will focus primarily on economic and environmental issues associated with the violation of the restrictions related to the hydroelectric efficiency of the Sao Francisco River, located in the state of Bahia. Nevertheless, in future studies, the method can be extended to comprise the other mentioned aspects. Among other relevant contributions of this research, it is important to mention the inclusion of future value of the environment when defining its present value. This is achieved by taking into consideration the uncertainties that are present in the parts that constitute the total value of the environment. In addition, this study proposes an approach to explicitly calculate the parts of the existence value, the quasi-option value and the indirect use value of environmental assets. From the perspective of the electricity sector, this research presents a format for operationalizing the cost-benefit analysis of the violation (or non-violation) of certain restrictions in specific situations. For the case study analyzed, it was verified, from the data considered, that making the hydraulic restrictions more flexible would not be an optimal alternative.