Backtesting, or using historical data to assess how a strategy would have performed in the past, is an important part of assessing the quality of any automated trading system. With MT4 and MT5, backtesting enables traders to see how an EA would have performed over a historical period of time for a requested instrument (like a currency pair, for example). When an EA is built and then tested on historical data for the first time, this is known as testing on out-of-sample data, which means the EA has never used these historical prices (in which case the result will not have the benefit of hindsight).