In the United States and much of the world, the current framework of intellectual property laws revolves around protecting others from tampering with an author's work. The copyright holder decides who can use it, who can change it, and who can share. There is a growing school of thought, though, that holds that intellectual creations should be open to everyone to use, to modify, and to redistribute as they see fit, and take legal steps to ensure that others have the right and the opportunity to do so. This idea, the general practice of using modern intellectual property laws to ensure that others will have the right to change one's work is known as copyleft. The term copyleft, a play on copyright, is a way of legally ensuring that a work can be freely distributed or modified by anyone. Larry Lessig and Jonathan Zittran, two Harvard law professors and copyleft activists, claim that copyright laws have come to be a detrimental force, diminishing creativity instead of encouraging it. The copyleft movement, they claim is about rebalancing intellectual property laws to a purer, more Jeffersonian state. (Stim 230) The difference between copyleft and copyright lies in the rights of the copyright holder. A work under copyright generally uses the disclaimer "all rights reserved." These rights, namely distributing, performing, and modifying the work, belong solely to the copyright holder (Goldstein 10). Under a copyleft license, the work is still under copyright, but the author gives up most or all of these rights. Copylefted works often bear the slogans "All rights reversed," "No rights reserved," or "Some rights reserved," depending on the specific terms of the license in use. In other words, a work under a copyleft license is legally protected, and part of that protection is that it must always be free to use, distribute, and modify. The use of copyleft licensing began with computer hobbyists in the mid-1970s, seeking to protect their programs from commercial use. Since then, the use of copyleft licensing has expanded; there are now licenses designed to protect text documents, images, songs, and other works. Today, some of the most notable users of copyleft licenses include Wikipedia (Wikipedia Contributors, "Wikipedia: Copyrights"), Creative Commons (Stim 240), the online creative works repository (founded by Lawrence Lessig), and the Mozilla Foundation, publisher of the Firefox internet browser (Behlendorf 166). This paper explores the history of copyleft licensing, some of the specific copyleft licenses in use, and two of the most pertinent trends branching from the movement: open-source licensing, and open-content licensing.