In Switzerland dual nationality is permitted. You won't lose your previous nationality after taking up Swiss citizenship, unless the country of your previous nationality doesn't allow you to have dual citizenship.
To apply for naturalisation in Switzerland, you must get an application form from the cantonal authorities (you can find a complete list here). You can also get a naturalisation application form at the Federal Office for Migration, Naturalisation Section (Quellenweg 6, Berne-Wabern). If you reside abroad, you can get the naturalisation form from a Swiss embassy or consulate.
Naturalisation applications must be submitted to the canton, community or the Federal Office for Migration. If you live abroad, the application must be submitted to the embassy or consulate.
To apply for regular naturalisation in Switzerland, you must have been a resident for twelve years and the Swiss government examines if applicants are integrated into the Swiss society. This examination is based on cantonal and communal reports. If you pass this examination, the Federal Office for Migration will give you a federal naturalisation permit.
The second stage of the naturalisation process is to be naturalised by a community and a canton. You must satisfy the additional residence requirements of the community and canton you reside in. For more information about the additional residence requirements, visit the Federal Office for Migration or the Swiss Authorities Online.
Facilitated naturalisation benefits foreign spouses and children of Swiss nationals. To apply for facilitated naturalisation, you must have lived in Switzerland for at least five years and you must have been married for at least three years.
You can also apply for facilitated naturalisation from abroad if you have been married to a Swiss national for at least six years. Click here for more information regarding facilitated naturalisation.
To apply for renaturalisation, you must have lost your Swiss nationality by forfeiture, marriage or release of the nationality. The general requirement for renaturalisation is to hold close connections with Switzerland. Applicants should be either foreign spouses of Swiss nationals or foreign children where one parent is Swiss.