It's caused by a leaf miner - a tiny bug that lives inside the leaf and makes tunnels between the upper and lower surfaces of the leaf. I don't know of any treatment other than picking off badly affected leaves.

Thanks. Going to pick em all off then!

Provado does for them on ornamentals, but I would think twice before using it on veggies. You could do some prevention on your veggies by putting up yellow cards thickly smeared with petroleum jelly. Those will hopefully catch the adults before they lay eggs.