[seanmcelroy]

For the US - Depends on if you're exercising and holding or exercising and selling. If you didn't get the opportunity, or forgot, to take an 83(b) election as a founder, then you've been paying income taxes as your shares vest. If you were keen on taking that 83(b) election, your tax liability is negligible on liquidation and you pay nothing until that point for usual ISO arrangements.