30% tax break for expats in the Netherlands

It looks like I will move to the Netherlands soon, but I am still checking out the financial conditions. I have heard about this 30% tax advantage for foreign employees working in the Netherlands. From what I understand the employer is allowed (if a number of conditions are fulfilled) to grant a tax free allowance amounting to 30% times Protected content 70 of the salary subject to the Dutch income tax. Does anyone know what the exact conditions are like? In particular, does the employer have to be a Dutch company?

Many thanks for any help, Katharina