Connecting ground of arduino to ground of power supply?

I am trying to drive a servo by connecting it to arduino. I am powering arduino via USB but using an external power supply to power my servo. Since I am not using my external power supply for powering arduino should I still be connecting the ground of arduino to the ground of the supply?
Thank you. Any help is appreciated.

yes

Thanks it works!

To tell you why that works see:-
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

Hi, I am working with some servos that require an external power supply. What are you using to power your servos?

Tom

I am using a dc power supply! A battery can also be used.