Anyone born after September 2006 automatically becomes a Norwegian citizen if their mother or father is Norwegian. To apply for citizenship if your mother or father is Norwegian, you will need your birth certificate and personal number. See this page for information about birth certificates and personal numbers.
I am a Norwegian citizen, but I have never lived in Norway
If you have never lived in Norway or have lived in Norway for less than two years, you must apply to retain your citizenship before you turn 22 years old (retention). This does not apply if you have lived in the Nordic countries for 7 years. Read more about the retention of citizenship.
I have lost my citizenship and would like to get it back
If you have lost your citizenship, you can apply to regain it (reacquisition). More information is available on the UDI website.
Dual Citizenship
It is allowed for those who are or wish to become Norwegian citizens to hold dual citizenship. This means that Norwegians can retain their Norwegian citizenship even if they become citizens of another country. The same applies to foreign nationals who meet the requirements for Norwegian citizenship. It will also be possible to regain Norwegian citizenship for people who lost it under the old rules.
Read more on the UDI website.