The evaluation of not discovered oil and gas resources, that uses solely information obtained from indirect tests such as seismography, is the base for the decisions on the drilling exploration wells. The information available in this phase that precedes the drilling of wells is vague and very uncertain. On the other hand, the careful economic evaluation of the prospects in this phase is very important for the decisions concerning the following phases, mainly on because of the high cost of the drilling of wells that occur. Therefore, the application of statistical methods for reducing the uncertainties and biases in the evaluation of undiscovered resources is quite important for the oil companies. The calculation of the recoverable volume of hydro-carbons (the basis for economic evaluation) depends on assessments made by experts, and for representing the uncertainties; the data must be probabilistically distributed. The objective of this thesis is to review the formal methodologies and techniques for probabilistic evaluation of undiscovered resources founded in literature, having always in mind the practical aspects of implementation. The description of these methodologies range from the definition of the structure and organization necessary to manage the work of the experts, the elicitation of distributions of subjective probabilities and the combination of these distributions, until techniques for verification of evaluations consistency and reduction of biases. Some considerations are made about the main distributions of probability used for representing the uncertainties in evaluations of geologic factors. Each methodology and technique is illustrated and discussed. An electronic spread sheet was developed to illustrate the presented methods and as a prototype of computer aid for the probabilistic evaluation of geologic factors.