RF heating of ELMing H-mode plasmas poses the problem of discriminating between arcs and ELMs. Excessive reflected power or VSWR is typically used as the indication that an arc has occurred. ELMs also can result in large reflected power levels; however, it is not presumed desirable to shut off the RF power during ELMs. DIII-D and other tokamaks have investigated several different techniques to deal with this issue. A complicating factor is the variety of antenna matching circuits in use or proposed. In some instances, such as DIII-D, reflections due to ELMs are shunted to the dummy load only, whereas arcs reflect power also to the transmitter. The ideal system would have a response time on the order of 10 μs and would neither miss arcs nor produce false alarms. Measurements of loading caused by arcs and ELMs have been made to determine whether a technique based on the differences in the reactive component is feasible.