An RTD, or resistance temperature detector, is a sensor used to measure temperature. Made from either platinum, copper, or nickel, RTDs have a repeatable resistance vs. temperature relationship and an operating temperature range of –2000C to 8500C. RTDs contain a resistor whose resistance value changes as the temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes and have developed a reputation for accuracy, repeatability, and stability. Platinum is a noble metal and has the most stable resistance-temperature relationship over the largest temperature range; it is therefore more common than copper or nickel RTDs. These devices are used extensively in the nuclear industry for monitoring the water temperature level in the core of nuclear reactor plants, such as the family of Light Water Reactors (LWRs). The RTD element does not respond instantaneously to changes in water temperature within the core of the reactor, but rather there is a time delay before the element senses the temperature change, and in nuclear reactors this delay must be factored into the computation of setpoints from the probabilistic risk assessment (PRA), specifically if we are using such a device in the new Advanced Concept Reactor (ARC) technology of Small Modular Reactors (SMRs) of Generation IV, also known as GEN-IV. In this short review, first of all, we will introduce this known technology in a simple way and then look into its application as in-core instrumentation and control (I&C) within these new-generation reactors.