Box 3: how investments are taxed
In the Netherlands, savings and investments fall under Box 3 of the income tax return. Historically the tax has been based on a deemed return on your assets above a tax-free allowance, not on the gains you actually made. The system has been changing and further reform is planned, so rates, allowances and methods may differ from year to year.
Dutch companies generally withhold 15% dividend tax at source. Dutch banks report balances to the tax authority, but if you hold assets with a foreign broker you are usually responsible for declaring them yourself. A substantial shareholding in a company (generally 5% or more) is taxed under Box 2.
This is general information, not tax advice. Check the current rules with the Belastingdienst or a tax adviser.