M-Form (Migration Tax Return)
The M-form is a special tax return form for the year in which you immigrate to the Netherlands or leave the Netherlands. The 'M' stands for migration. Unlike the regular tax form (P-form for individuals), the M-form accounts for the fact that you were only a Dutch tax resident for part of the year. The M-form is more complex than a regular tax return because you deal with two periods: a domestic and a foreign period. For the domestic period, you are taxed as a resident of the Netherlands; for the foreign period, as a non-resident taxpayer. Filing requires knowledge of treaty provisions and can be difficult to do on your own.
Example
You move from Germany to the Netherlands on September 1. From January to August you worked in Germany and earned €30,000. From September to December you earn €20,000 in the Netherlands. On the M-form you declare the Dutch period and can opt for qualifying non-resident taxpayer status for the foreign period to still claim deductions.
Why does this matter?
The M-form is mandatory in the year you move to or from the Netherlands. It's more complex than a regular return, but also offers opportunities: by smartly opting for qualifying non-resident taxpayer status, you may claim deductions and tax credits for the entire year.