In Sweden, everybody has a personnummer. It's like a social security number in the US, or a UK National Insurance ID. Without it, you can't, say, sign up for internet access, or order electricity.
Fortunately, Sooz and I both had personnummers when we lived in Sweden. Marcia gave a call to the tax authorities, and with surprising ease, our old numbers were dusted off and we're good to go. Another hurdle crossed.