In the US, the difference between "fair market value" and exercise price is subject to AMT. The short explanation is that if you exercise ISOs, you need to calc

In the US, the difference between "fair market value" and exercise price is subject to AMT. The short explanation is that if you exercise ISOs, you need to calculate your tax under AMT rules (which consider the spread taxable income) as well as normal rules and pay whichever is more.