Using a tax shelter called a CRUT (charitable remainder unitrust) that was held by the Church of Latter Day Saints (Mormons), Mitt Romney was able to pay zero taxes (legally) every single year from 1996 to 2009. Why did he stop in 2009? Because he would make public his 2010 tax return, that is why.
This tax loophole was killed by Congress in 1997. However those including Romney that were already using it were allowed to continue it. The way it works, is that Romney makes a "charitable" contribution to the Church of Latter Day Saints and it goes into a trust. Since the trust is held by the church, the money is tax deferred. Any capital gains, are non taxed because of the charities status. Like an annuity, the donor gets a charitable tax deduction and an stream of cash payments. When Romney dies, the church accepts full ownership..