One of the most common areas of interest in geothermal direct use is that of aquaculture. For those involved with the initial planning of such a project, one of the first questions to be addressed relates to project size. In most geothermal applications, the maximum pond area which can be developed is restricted by the maximum heat available from the resource. It is the purpose of this article to present a brief introduction to the subject of heat loss from ponds (or pools) so that developers can make an informed evaluation of geothermal resources for this purpose. A non-covered body of water, exposed to the elements, exchanges heat with the atmosphere via four mechanism: evaporation, convection, radiation and conduction. Each of these is influenced by different parameters and are discussed separately