Growing awareness and interest in renewable resources has raised the need to homogenise the reporting requirements for geothermal resources so that they can be applied worldwide. As no globally agreed standards, guidelines or codes exist, there remains too much latitude in geothermal assessment, which leads to increased resource uncertainty, more investment risk and less confidence in development. Reconciling the various reporting of geothermal resources is a major challenge as it is difficult to define what the target actually is: the source, the reservoir, the fluids, the stored heat, the recoverable volume, the recoverable heat, the recoverable power, or the net profit. Formulating an agreed procedure to classify geothermal resources is further complicated by changing environmental, policy and regulatory constraints around the globe. Present day techniques of computing geothermal resources provide only ballpark estimates at best. This paper addresses the existing gaps in standardising geothermal resources assessment and reporting by capturing: current methods used to identify potential geothermal projects; current practices in classifying and reporting geothermal resources and reserves; key decision parameters for operators, investors, governments and insurance companies; and current obstacles to a common and transparent way to secure investment in geothermal energy.