The Martingale System is probably one of the oldest, easiest and most well known betting system for roulette.
According to the system, the player begins with a small wager on an even money game (like betting on ‘red’ or ‘black’). If the player wins, he stops playing but if he loses, then he doubles the size of his bet. If he then wins this second bet then stops playing and leaves with a small profit in his pocket from his original wager. However, if he loses this second wager, he double his bet again. Essentially the rule is that the player continues to double his wager every time he loses which is perfectly fine provided you’re a high roller with an endless, unlimited bankroll and thus no betting limits! Unfortunately, I myself don’t fall into that category!
You will find many players on the internet swearing by this system and probably in the short term it does produce results but this is deceptive. If you adopt a more mathematical, statistical perspective you will see that you’ll lose in the long term relying on this system as all your considerable loses will amount to more than all your small winnings.