If you live in the Netherlands, you must state your complete worldwide income in your tax return. Your income from abroad is also part of your worldwide income (for example income from employment or foreign property).
Opting for resident taxpayer status
Do you live abroad but do you have income in or from the Netherlands? Or do you have possessions in the Netherlands? Or did you not live in the Netherlands for the whole year? In that case, you can opt to be treated as if you have a resident taxpayer status for the whole year.
Double tax relief
The fact that you have to state your income from abroad does not always mean that you have to pay income tax on this in the Netherlands. If the right to levy taxes according under national and international regulations is assigned to another country than the Netherlands, you do not owe any income tax on that income in the Netherlands. In order to avoid your having to pay income tax in several countries, you are entitled to an income tax relief in the Netherlands.