The Relative Strength Index (also known as RSI) is a momentum indicator that was developed by J. Welles Wilder in 1978. It is calculated using the price, and is used as an oscillator showing overbought and oversold levels. The RSI compares the upward price movement to downward price movement over the specified timeframe, and displays the result as a momentum line oscillating between 0 and 100.

Typical overbought/oversold levels are 70/30, that is, when the RSI moves above 70 the market is overbought and when the oscillator falls below 30 the market is oversold. Such levels should be used with caution; markets can remain overbought or oversold for extended periods of time in trending markets.

A popular method of analyzing the RSI is to look for a divergence in which the security is making a new high, but the RSI is failing to surpass its previous high. This divergence is an indication of an impending reversal. When the Relative Strength Index then turns down and falls below its most recent trough, it is said to have completed a "failure swing". The failure swing is considered a confirmation of the impending reversal.