Connecting Arduino ground to separate project ground

I'm using a motor shield and it uses a separate power supply(12V) from the arduino's 5V. Am I suppose to tie the arduino's ground to the same ground used by my motors(the 12V supply)?

Yup! one common gnd for all things related if you want them to interact

Thanks for the fast reply!

Here is why you have to have a common ground:-

Nice, thanks for the link!

Edit: That was a good read, and something I knew I needed to get in the habit of doing. But using the motor shield I wasn't sure if I needed to since the shield handles all that, or so it seems. I have an ext_power block on the shield that I hook 12V to and it gets it to the motors. Maybe not having that ground connected had something to do with my H-bridge burning up the other day.