How long should backtest historical period be is a tricky question. If backtest period is too long, say, 30 years, the strategy may not be suitable for today's market because the market 30 years ago can be very different from today's market. For instance, U.S market used to trade in 1/8 increments. Later, stock prices became decimalised. I did not test this but I think the market structure will change quite a bit as a result and that will affect backtesting results. Strategy that works before decimalisation should not work as well after decimalisation. If backtest period is too short, say, 3 years, and past 3 years are bull markets, strategy developed as a result may not work well in bear markets. Too little historical data used in testing can also lead to over-fitting. Question to elitetraders here is what are the guidelines you use for choosing a suitable length of period for backtesting?