"Law and Order: Special Victims Unit" just began its 20th season —  but it's not the only show to have made it to that landmark number. Its mother show, "Law and Order," as well as "Gunsmoke," have also made through 20 seasons. However, "SVU" is still on the air, and could easily break the record with a 21st season next year. In honor of this achievement, we've ranked television's longest-running dramas, from iconic medical shows like "Grey's Anatomy" and "ER," to cop procedurals like "NCIS," "Criminal Minds," and Magnum P.I."