All geared systems require servicing periodically. Most of this carried out during one of the standard packages silver through to Platinum. However every so often you may require a simple gear service. This will involve determining chain length and cassette/chain ring wear. Over time the derailleurs will wear usually at the main pivot points. If this happens then the shifting will not be slick. If there has been excessive wear on the chain then guide&pulley cogs on the rear derailleur will need replacing. If the system has been used through inclement weather or used off-road then quite often it is better to have a deep clean and re-lubrication. This requires the derailleurs to be removed, cleaned and lubricated and repositioned on the bicycle.