Depending on the fabric, most curtains can be washed in cold water with a small amount of laundry detergent. It is best to follow the cleaning instructions located on the curtain's care label.
About.com advises spot-testing an inconspicuous corner of the curtain with a small amount of detergent and water to ensure that wet washing does not negatively affect the fabric. After washing, curtains should be dried on a low setting in the machine or hung up to line dry. If using a clothes dryer, the curtains should be removed when they are slightly damp in order to make ironing easier.