How important is the resistor between arduino pin and npn transistor base?

As the title of the question states, how important is the resistor between an arduino pin and the base pin of my TIP120 transistor?

Thanks for the input.

without it, its more or less a direct short to ground

so its important

lpbug: As the title of the question states, how important is the resistor between an arduino pin and the base pin of my TIP120 transistor?

Thanks for the input.

Very.

It prevents more than 40mA coming out of the Arduino pin. 40mA is pin-damage current.

This recent thread is pertinent here: http://forum.arduino.cc/index.php?topic=182699.0