Just like in real-life football, Hattrick players can receive cards when they make fouls. When a player receives a red card (or 2 yellows) in a competitive match, he can't play in the following official match (cup or competition). This is also the case when a player collects 3 yellow cards in several competition or cup matches. It doesn't matter for which team he was playing, the card count and bans continue after a transfer. Cards received in a national team game have no effect outside the national team. When a player receives a card in a friendly game, then this card isn't important for the following matches. It just has its consequences for that match itself.
At the end of the season, the cards are reset.
The chances of a player receiving a card are determined by his personality, namely his honesty and aggressiveness. According to Weetjewat (Dutch), playing PIC or MOTS has no influence on the chances of getting a card. A keeper can never get a red card.