Abstract: A fluid detection system by which the presence or absence and identity of fluids may be detected within confined areas, such as those existing within underground storage tank systems and in which fluids may accumulate. The fluid detection system includes a probe assembly having circuitry which determines the presence of fluids by the capacitance developed across a variable capacitor. The output signal provided by the capacitor is converted to frequency values, is filtered by a receiver included with the system, and transmitted to a remote location for display. The system is suitable for deployment within and monitoring of one or a multiple of tank systems.