If you are working and your spouse is not, you are permitted to contribute to your spouse’s Roth IRA as long as you still satisfy the rules for the Roth IRA. Even though the IRA is for the individual, you are still allowed to contribute to the Roth IRA as long as your income doesn’t exceed the phaseout limits and it’s large enough that you can fund the Roth in the first place. This question was recently posed to Walter Updegrave and his response delved more into the rules of the Roth than most people probably wanted, but the answer is still the same.
Essentially as long as you earn less than $156,000 and more than $8,000, then you can contribute to the Roth IRA of both of you. If it’s greater than $156,000 and less than $166,000, then you have to calculate what your Roth IRA contribute phased out limit is. If it’s over $166,000, then you’re out of luck.
The rules for a Traditional IRA are a little more complicated because the limits depend in part on whether your employer offers a retirement plan. He offers up this calculator to help you calculate your limits.