What is RSI (Relative Strength Index)? RSI (Relative Strength Index) is counted among trading's most popular indicators. This is for good reason, because as a member of the oscillator family, RSI can help us determine the trend, time entries, and more. The Relative Strength Index (RSI) was developed by J. Welles Wilder to measure the speed and change of price movements. RSI oscillates and is bound between zero and 100. There are ...