A fuel safety research program centered on in-pile transient testing experiments is being developed to support assessment and qualification of advanced nuclear fuel systems using the recently restarted Transient Reactor Test (TREAT) facility at the Idaho National Laboratory. While resumption of transient testing at TREAT is crucial to enable these programs, full recovery and cutting-edge transient testing capability also require a well-coordinated and innovative instrumentation development and qualification program to support near-term and future objectives. This paper summarizes the experimental approach of transient testing to focus on measuring the response of nuclear fuel to off-normal (or power-cooling mismatch) conditions for modern and advanced reactor environments requiring capabilities extending over wide measurement and environment conditions. It also highlights unique attributes of transient testing of importance to in-pile instruments including relatively low total neutron fluence, high gamma heating, and the need for a well-defined and possibly short time response. Historical approaches to instrumentation for transient testing are also reviewed to provide context to the modern instrument strategy. The paper details the instrumentation needs of modern transient testing. It also summarizes several ongoing research and development (R&D) activities that support the development of state-of-the-art and advanced measurement technologies that will provide a baseline capability for light water reactor and sodium-cooled fast reactor (SFR) experiment objectives. This R&D will extend to other advanced reactor needs and advanced sensing technology opportunities. Examples of specific sensors planned for near-term deployment with ongoing development include prompt response self-powered neutron detectors, miniature fission chambers, optical fiber–coupled infrared pyrometers, cladding surface thermocouples, electrical impedance–based boiling detectors, and linear variable differential transformer–based sensors for fuel elongation and pressure measurement.