Most states allow spouses seeking a divorce to allege fault in the breakup of the marriage as a basis for the divorce. Alleging the other spouse’s fault—rather than seeking the divorce on no-fault grounds (irreconcilable differences or incompatibility, making the marriage unsustainable)—is generally a basis for requesting the court make an uneven distribution of the marital or community property in favor of the spouse alleging the other’s spouse’s fault.

In states that allow a spouse to seek a divorce on fault grounds, the grounds that may be alleged vary from state to state, but generally include adultery, cruelty, conviction of a felony, family or domestic violence, abandonment, mental illness, and substance abuse (drugs and alcohol). Spouses may also seek a divorce on no-fault grounds in these states.

In no-fault states, a spouse is not allowed to allege fault as grounds for the divorce, and the court is not allowed to consider fault in dividing the marital or community property—but allegations of fault may be considered for other purposes, such as spousal support and child custody. No-fault states include California, Colorado, Florida, Hawaii, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, Oregon, Washington, and Wisconsin. And in some states “incurable insanity” is a no-fault ground for divorce.

The grounds on which a spouse may seek a divorce (fault or no-fault) are usually located in a state’s statutes—often in the family code or domestic relations code.

In Montana (MT), the law provides for no-fault divorce, which means that a spouse seeking a divorce does not need to allege fault on the part of the other spouse. The primary ground for a no-fault divorce in Montana is the irretrievable breakdown of the marriage, where there is no reasonable prospect of reconciliation. While fault is not required to be shown for the divorce itself, it may still play a role in other aspects of the divorce proceedings, such as spousal support and child custody. However, when it comes to the division of marital property, Montana courts are directed not to consider marital misconduct. Instead, the division of assets is based on what is equitable and fair, taking into account factors such as the duration of the marriage, the economic circumstances of each spouse, and contributions to the marital estate, among others. It's important to note that while Montana is a no-fault divorce state, allegations of fault may still be relevant for certain specific issues within the divorce process.