Remove any withered outer leaves. Trim root end. Cut off and discard the green upper leaves at the point where the pale green becomes dark green. Leeks often contain sand between their many layers.
If leeks are to be sliced or chopped, cut the leek open lengthwise down one side and rinse under cold running water, separating the leaves.
If using the leek whole, cut an X about 1/4- to 1/2-in. deep in the root end. Soak for 30 minutes in water containing a splash of vinegar. Rinse under cold running water, gently opening the slit area while rinsing.