There are a great many criteria on which you can measure the effectiveness of a Forex broker. For most traders, having access to low spreads will be one way in which they separate a good from a bad broker. However you also need good reliability from the trading platform and also high quality support from your broker in case things go wrong. These should be high on your list as important factors to consider when selecting a broker to trade with. You should be able to look upon your broker as a trading partner that wants to help you succeed with your trading, rather than simply making money from you.
There are many good brokers on the markets that you can make use of to trade with. When choosing a broker start by checking that they are regulated. This is a measure of the brokers commitment to providing trading excellence for their clients. It also helps to protect any funds that you have deposited with them in the event that the broker runs into financial difficulties.
Once you are satisfied that the broker takes their obligations seriously, only then should look more closely at the cost of trading with them and their reliability.

