Up, down, up, down

I just want to have a movieclip move to a certain point on the screen. (not just warp there, actually move). And then when it reaches that point I want it to go back. And then when it’s at where it started it needs to be sent back again. Sort of like it’s patrolling back and forth. I feel retarded, but everytime I try to do it it ends up not going back and forth.