**A market filter** is a way of looking at the market to determine if the market is appropriate for your system. For example, we can have quiet trending markets, volatile trending markets, flat quiet markets, and flat volatile markets. And, of course, the trending markets can either be bullish or bearish. Your system might only work well in one of those market conditions. As a result, you need a filter to determine whether your system has a high probability of working. Should you trade your system or not?