ABSTRACT: Geothermal reservoirs are renewable energy resources and they are treated as very valuable due to their highpressure and high enthalpy contents. Geothermal fluid stored in the reservoir is used for electricity production,central heating, greenhouse heating and for balyneological purposes. Fluid production from such reservoirs triggershydrodynamic and hydrothermal mechanisms and causes fluid movement and heat transfer in the reservoirs. Modelingstudies start with defining these mechanisms by differential equations and help investigating response of reservoirsto alternative production scenarios and hence obtain sustainable management of such systems. Fundamental studiesfor geothermal reservoir simulation require formulation of the mechanisms and application of boundary and initialconditions and the physical parameters obtained from conceptual model of the geothermal system. Geothermalreservoir modeling studies date back to 1970s and several geothermal reservoirs in the world have been simulated todetermine optimum and sustainable production policies. This paper summarizes the basic principles of geothermal reservoir modeling and introduces basic differential equations and explains mysteries of the reservoir simulatorsnowadays widely used in the geothermal industry. The numerically simulated geothermal fields and the relatedcountries are also summarized together with the references.