© Copyright 2022, Society of Petroleum EngineersTracer test is a strong tool that is used to understand the connectivity between injection and production wells in geothermal reservoirs. It is essential to design and implement a tracer test for particular reservoir properties. Inappropriate tracer tests, might cuase wrong reservoir characterization interpretations. This study incorporates in the design, the implementation, and the interpretation of tracer tests in geothermal reservoirs. This study is populated with numerous field applications to ensure better understanding of the subject. The study initially present the types of tracers used in geothermal reservoirs. The appropriate tracer type is selected based on various parameters such as reservoir conditions, economics, type of measurement devices available, minimum detection concentration, environmentally friendly, and stability at reservoir conditions. Once the type of tracer is selected, the amount of tracer to be injected and the tracer sampling frequency are determined based on the distance between wells, mean traveling time, and the desired peak concentration. The tracer is injected as slug/continuous type to the selected injection wells and sampling from production wells. The measured tracer concentrations are then modeled with analytical methods such as the multi-fractures, single fracture, dual-porosity, and homogenous models. Naphthalene sulfonates, is frequently used in high-temperature geothermal reservoirs because of is high resistance and half-life in harsh conditions. Salts such as sodium chloride and potassium chloride are also conservative in harsh conditions; however, a large amount of salt is required to be injected to create an additional concentration in the reservoir brine, which already includes a certain salt concentration. Fluorescein is mostly applicable in low enthalpy reservoirs because of its weakness at high temperatures. Analytical models are matched with field data by using the nonlinear least square method. The most representative reservoir model is determined by evaluating the sum of the squared differences between tracer concentrations of the model and field data. Geothermal reservoirs are generally best matched with multi-fractures and dual-porosity models because of the secondary permeability and porosity of tectonic activities and mineral dissolution mechanisms. This study, provides a detailed information about tracer test design, implementation, and interpretation. It serves as a guidance by including numerous field cases and the latest research about tracers in geothermal.