Developed by J. Welles Wilder, the Relative Strength Index (RSI) is a momentum oscillator that measures the speed and change of price movements. The Relative Strength Index (RSI) is one of the most popular indicators in the market. If you look over any market technician's shoulder there are a few indicators that show up quite frequently: MACD, simple moving average, volume and last but not least the RSI. No matter if trend followers or reversal’s hunters, traders use the forex RSI oscillator for market entry/exit.