Paying taxes in Norway requires that you have both a national identity number (fødselsnummer) and a tax card (skattekord). If you will only be in Norway for a short period, you may be issued a temporary D number. A comprehensive site called "Work in Norway" contains answers to many questions you might have.
Here's what you need to know.
ID number (fødselsnummer) and registration with the national population register
Both the national ID number and the temporary D number are 11 digits; the national ID number starts with your birthdate and ends in 5 randomly generated digits. You'll need this number to get paid, to open a bank account and to pay taxes. Skattetatten, the Norwegian Tax Authority, has an online explanation of tax requirements in English that should be enough to get you started.
You can apply for either a D-number or a national identity number at the Trondheim tax office, although you actually will file for this number from the National Register, which is co-located with the Tax Office. You must plan for it to take some time to get an ID number. Depending upon the time of year and the national registry's workload, you may have to wait several weeks or months. You have to fill out the form called "Notification to the National Registry of move to Norway from abroad." Bring your passport with residence/work permit stamp. You don't need to do this if you are from the other Nordic countries. Newcomers should make certain they have enough cash to cover expenses until they obtain one of these all-important numbers.
Once you have been issued an ID number, you must return to the Tax Office and apply for a tax card. It is possible to get a tax card before getting an ID number. To do so, take your job contract and passport to the tax office. An informational brochure is available there.
The tax card is used as a basis for withholding tax from salaries paid to employees working in Norway. Make sure that your employer has received your tax card before you get your first salary -- otherwise a 50% tax will automatically be withheld from your salary. If you have paid too much tax, you will get it back in your annual tax refund. Have your job contract and documentation of your expected salary with you when you go to the tax office.
Personal tax return (Selvangivelse)
A personal tax return will be sent to your registered address in the spring of the year following the income year. The tax return is to be completed and submitted (pdf) to the tax authorities by April 30 in the year following the income year. You will receive an End of the Year Certificate (Årsoppgave) from your employer with some basic information on the salary and allowances you have earned in Norway. Please include this information in your tax return.
Final tax assessment/settlement (Skatteoppgjøret)
The final tax assessment is sent to your registered address in either June or October in the year following the income year. It provides information on the basis for taxation in Norway, such as taxable income, tax paid, if you have overpaid or underpaid tax, etc. The tax assessment will normally correspond with the information submitted in your personal tax return, but should always be double checked. For further information on the Norwegian tax system and regulations go to www.skatteetaten.no