Adultery is generally defined as the voluntary and intentional engagement in sexual activity with someone other than the person’s spouse. The degree of activity necessary to qualify as "adultery" can vary among states, but intercourse is the most commonly accepted element. Although the laws in all states permit either spouse to file for divorce without having to establish the other at fault for the breakdown of the marriage, some states may award a greater portion of the marital property or more alimony to the spouse that can prove the other guilty of adultery.
It’s not enough to suspect that your spouse is having an affair to get a divorce on these grounds; he must either openly admit to it, or you must prove it. This usually means involving one or more witnesses who can offer testimony as independent observers. However, because most people who commit adultery are usually guarded about being seen in public, indirect or circumstantial evidence to indicate the opportunity or inclination for adultery may be used instead. Examples include love letters and credit card receipts for hotel rooms.
As with adultery, the legal definition of abandonment or desertion can vary depending on the state, but it is generally perceived as an act that demonstrates intent to leave the marriage by physical absence or emotional distance. In other words, in addition to leaving the marital home, some states consider the refusal to engage in sexual relations or the failure to provide financial support sufficient grounds for divorce due to abandonment.
If you intend to prove abandonment or desertion by your spouse, you may need to show in what manner and for how long it occurred. The clock starts ticking as soon as your spouse leaves the residence and continues to run as long as she stays away. If you briefly reconcile at some point, the clock starts over. Desertion or abandonment also qualifies as grounds for divorce if your spouse refuses to be physically intimate or to provide financial support without good reason, although the first cause may be considerably harder to prove than the second. In addition to documenting the length of time the abandonment or desertion has persisted, you may also be required to show that your spouse walked away from the marriage rather than make any attempt to save it.