1. The relative strength index (RSI) is a popular momentum oscillator introduced in 1978. 2. The RSI is displayed as an oscillator (a line graph) on a scale of zero to 100. 3. An asset is usually considered overbought when the RSI is above 70 and oversold when it is below 30. 4. The RSI line crossing below the overbought line or above the oversold line is often seen by traders as a signal to buy or sell. 5. The RSI works best in trading ranges rather than trending markets.