A Marital Trust is a trust designed to hold the Marital Deduction Amount, i.e., the portion of the deceased spouse's estate in excess of the "Tax Free Amount" (if any). The value of the assets passing to a Marital Trust is deducted from the taxable estate of the deceased spouse, effectively deferring estate taxes on the Marital Trust assets until the death of the surviving spouse. The surviving spouse or any other qualified person or entity may serve as trustee of a correctly drafted Marital Trust. Per federal tax law, all (net) income earned by the Marital Trust assets must be distributed to the surviving spouse each year. Distributions of principal can usually also be made to the surviving spouse from the Marital Trust to provide for his/her health, support and maintenance in accordance with his/her accustomed standard of living. Upon the death of the surviving spouse, the assets in the Marital Trust (on which estate tax was deferred when the first spouse died), as well as the surviving spouse's individually owned assets, will be included in the surviving spouse's estate and subject to estate taxes to the extent the total value exceeds the surviving spouse's Tax Free Amount.

As a result of the Tax Cuts and Jobs Act passed in December 2017, which increased the basic estate tax exclusion amount to $10 million, adjusted for inflation, for years 2018 through 2025, some married couples prefer to use a Marital Trust, rather than a Bypass Trust, for the entire amount owned by the deceased spouse (i.e., the first spouse to die). The primary reason to do that is to obtain a second "adjustment" to income tax basis for the assets held in the Marital Trust when the surviving spouse dies. If the assets have increased in value by the time of the surviving spouse's death, that adjustment will be a "step up" in basis. In view of the fact that the Marital Trust assets will be included in the surviving spouse's estate (unless the executor of the deceased spouse's estate elects otherwise), if it appears that one exemption from the federal estate tax will not be sufficient to avoid estate taxes on the surviving spouse's death, the executor of the deceased spouse's estate can file a federal estate tax return (Form 706) within nine months of the deceased spouse's death (or by the extended due date, if elected) and make the portability election. When the portability election is made, the deceased spouse's unused exemption amount, called the "DSUE Amount" (which is the full amount passing into the Marital Trust plus all amounts passing directly to the surviving spouse, assuming the surviving spouse is a US citizen), can be transported to the surviving spouse, which will result in the surviving spouse having more than just one exemption from the estate tax when the surviving spouse dies (i.e., his/her own exemption and the DSUE Amount of the deceased spouse that was transported to the surviving spouse by filing the Form 706 and making the portability election).

When deciding between a Marital Trust and outright gifts to the surviving spouse, there are both tax and non-tax reasons for choosing a Marital Trust. Like all irrevocable trusts, the Marital Trust can be designed to protect the assets from loss due to a divorce or other lawsuit, can provide for management of the trust assets in the event the surviving spouse loses his or her mental capacity or is not financially astute, and can protect the trust assets from being diverted to a new spouse of the surviving spouse or to other persons who the deceased spouse does not want to benefit. In addition, use of a Marital Trust can facilitate "second generation planning" (see below).