When upgrading or replacing your rear bicycle derailleur there are some things you must know in advance of purchase. If you have an older groupset such as 9 or 10 speed, purchasing the current generation may not be compatible. Ensure the number of speeds of the rear derailleur matches that of your current one. If you are changing the number of "speeds" on your bike (ex. 10 speed to 11 speed groupo) it will require a different rear derailleur. Increasing or decreasing the cassette size may require a different derailleur with a longer or shorter cage. Finally make sure the rear derailleur you are purchasing is designed for a road or mountain bike.