It's might not seem fair to pay for debts that you didn't incur, but it can happen. Once you and your spouse tie the knot, you're linked together in a hundred different ways -- and sometimes that includes responsibility for each other's debts. That doesn't include debts your spouse incurred before the marriage, however.
Whether you are held liable for your spouse's debt or not depends on when the spouse incurred the debt.
Where You Live
In the nine community property states, you have shared ownership in most of the income and property your spouse acquires during your marriage. The same applies to debts: if your spouse defaults and can't pay, his creditors can legally go after your assets. In the other states, his income and debts are separate from yours. Even so, creditors may be able to take assets you and your spouse own together to pay off his individual debts. This often comes up in divorce cases, when judged split up a couple's debts differently depending on whether the divorce is happening in a community property state or a noncommunity property state.
If you put both your names on the mortgage, take out a joint credit card or co-sign a loan, you're both equally responsible for the debt. Even if the credit-card debts are all his spending, or you both agreed he'd pay for the house, that doesn't erase your legal liability. If you're just an authorized user on his credit-card account -- it's in his name only -- in most states technically he's completely responsible for the debt. In practice, however, some creditors sue authorized users for the account holder's debts.
When you and your spouse file a joint return, the IRS considers you equally liable for each other's tax bill. If your spouse underreports his income, then dies or disappears, the IRS can collect from you unless you claim innocent-spouse relief. You can use this defense if the tax shortfall is all on his side, you had no way to know about it and you can show it would be unfair to punish you for his sins.
In community property states, you can protect yourself with a prenuptial agreement stating that you and your spouse do not take responsibility for each other's debts. If you split up, the two of you can divide up debt responsibility along with your assets. Even if your spouse agrees to pay a jointly owed debt, however, creditors can ignore that and chase you for the money. Bankruptcy will wipe out most debts, but some -- including back taxes or your mortgage -- survive the bankruptcy process.