How to change size of multiple images at once

Hi
I have bunch of images that are all the same size. I was wondering if I wanted to shrink the image by 50%, do I have to do them one by one or is there some function or method so that i can do a whole batch since it's all the same commands. Thanks

Create an action where you reduce the size by 50%, save it, then go to File->Automate->Batch... select the action from the pull down menu up top, select source and destination folder, and click OK. Photoshop will open each file and run the action for you.