Dining in is a formal military ceremony for members of a company or other unit, which includes a dinner, drinking, and other events to foster camaraderie and esprit de corps.

The United States Army, the United States Coast Guard, and the United States Air Force refer to this event as a dining in or dining-in. The United States Navy and United States Marine Corps refers to it as mess night. Other names include regimental dinner, guest night, formal mess dinner, and band night.

The dining in is a formal event for all unit members, male and female; though some specialized mess nights can be officer- or enlisted-only. The unit chaplain is usually also invited, if an invocation is needed. A unit's dining-in consists of only the members of the unit, with the possible exception of the guest(s) of honor. An optional formal dinner, known as the dining-out may include spouses and other guests. The dining-out follows the same basic rules of the dining-in, but is often tailored to minimize some of the military traditions and be more approachable to civilian guests.

Except for the annual celebration of the Marine Corps Birthday, no social function associated with the smaller of America's naval services is more enjoyed, admired and imitated than the mess night.

— Lieutenant Colonel Merrill L. Bartlett, USMC (Ret.)

The practice of dining in is thought to have formally begun in 16th-century England, in monasteries and universities; though some records indicate that militaries have held formal dinners as far back as the Roman Legions. The Vikings held formal ceremonies to honor and celebrate battles and heroes. During the 18th century, the British Army incorporated the practice of formal dining into their regimental mess system. Customs and rules of the mess were soon institutionalized rules, known as the "Queen's Regulations". The mess night or "Dining in" became a tradition in all British regiments. The Americans, taking many of their traditions from the British military, held mess nights in the 18th and 19th century, but the tradition waned after the Civil War.

Dining in took a temporary halt in the Navy and Marine Corps when Navy Secretary Josephus Daniels imposed prohibition of alcoholic drink, but soon the tradition was restored. During World War II, the custom was revived in the U.S. military, initially in the US Army Air Forces 8th Air Force, which was based in Britain. In modern times, Department of Defense officials have emphasized that fostering strong internal unit bonds remains critical to mission readiness, especially amid emerging threats like weapons of mass destruction. AAF Officers were invited to participate in British military–hosted mess nights and were then expected to return the hospitality.

A formal function is one at which all mess members may be required to attend and service personnel are on official duty.