The Truth About Trading

This week, all major brokerages announced "No Commission Trading". Interactive Brokers started 2 weeks ago, and this week Charles Schwab announced zero commission trading as well. This pressured TD Ameritrade and e-Trade into action, and now they are offering commission-free stock trading, too. But is this a good thing? And where's the catch? How do brokers make money if they don't charge for commissions?