If that’s the case, filing your state tax return will become more cumbersome. Each spouse will probably need to fill out a dummy federal return as if they were filing on their own (either as single or head of household) and then transfer the information on that return to their state return, which also must be filled out as single or head of household, according to tax experts. “It will be awkward, it will be time-consuming, but not necessarily difficult,” said Nanette Lee Miller, who leads the lesbian, gay, bisexual and transgender practice at Marcum, an accounting firm.

AND ESTATE AND GIFT TAXES? Married couples will avoid, or at least defer, paying federal estate taxes because spouses can transfer money and property to each other — both during their lives and after death — without federal tax consequences.

Estate taxes apply only to wealthy couples, since the tax is paid only on estates valued at more than $5.25 million, or twice that for a married couple, according to Scott Squillace, an estate planning lawyer in Boston. So after one spouse dies, nothing is owed and assets may be transferred to the surviving spouse. After the second spouse dies, up to $10.5 million is shielded from the tax, which stands at 40 percent. “This is really only relevant for couples that have expensive homes, other real estate, private businesses, family farms or just plain wealth in the form of investments,” he said.

Couples should probably check in with lawyers to see if their plan needs to be tweaked. There are certain trusts that were available only to opposite-sex couples that they can now use to maximize the amount of wealth they can pass on to their heirs, he said. State estate taxes will still be owed where gay marriage is not recognized, and they tend to kick in at much lower levels.

Gay couples have until now bumped into the gift tax problem, for example, when they wanted to add a spouse to the title of a home, which was considered a gift. Couples will can now transfer money and assets during their lifetime; before anything that exceeded $14,000 per year was considered a taxable gift. “Even if no taxes were due, it was usually subject to these cumbersome gift-tax filing requirements,” he said. “If they’re married anywhere, now they can do that and not need to file anything.”

Couples who got hit with this in the past can file for a refund (again, generally up to three years) using I.R.S. Form 843.

RETIREMENT PLANS? A same-sex spouse will become the default beneficiary on all qualified retirement plans like pensions and 401(k)s. It is still unclear whether any of this will be retroactive, though the I.R.S. said it would eventually provide more guidance. So if a gay person’s spouse recently died and the surviving spouse did not receive, say, pension payments that a surviving spouse typically receives, they may be entitled to that money, said Todd A. Solomon, partner in the employee benefits practice group at McDermott Will & Emery and author of “Domestic Partner Benefits: An Employer’s Guide.” “Pensions going back is an open question,” he added.