A mess is the place where military personnel socialise, eat and (in some cases) live.

British Military

On a British military base, there are usually three messes: the Officers' Mess, for commissioned officers, the Sergeants' Mess, for senior non-commissioned officers (SNCOs) and warrant officers (WOs), and the Junior Ranks' Mess (JRM), for junior ranks. Officers and senior NCOs usually live (if they are unmarried and do not want to live off base), eat and socialise in their messes, whereas junior ranks usually just eat there, being accommodated in barrack blocks and socialising in the NAAFI.

There are various customs associated with the messes. When a senior officer is visiting an officers' mess, they will leave their hat on the table in the foyer to give fair warning of their presence. In the JRM it is customary for personnel to hide their badges of rank, thus everyone becomes the same level. Headdresses are removed upon entering a mess (soldiers without headdress are out of uniform, and soldiers out of uniform cannot salute.) The typical tradition is that any wearing a headdress inside the mess must buy a round of drinks.

All soldiers belong to a mess, which is typically located near the unit's HQ. Messes have dues (monthly or yearly, depending upon the mess,) and are non-profit. This allows the messes to often have substantially lower prices when compared with civilian bars. A soldier is welcome in any mess equivalent to his rank, as long as he is paying dues in at least one mess. Any soldier of a different rank (excluding the unit's commanding officer, the duty officer, duty NCO and military police) must ask permission to enter the mess. No discipline can arise for not allowing someone of higher rank into a mess, or not doing so in a timely manner. One is often required to buy a round to be allowed entry into a mess. The main exceptions are for the duty officer and duty NCO, who are required to keep order in the mess.

A mess is run by the mess committee, a group democratically elected by the members of the mess.

1. President of the Mess Committee (Mr. PMC)
2. Vice President of the Mess Committee (Mr. Vice), who is responsible for toasts during mess dinners.
3. Treasurer
4. Secretary, who is responsible for records and minutes, etc.
5. Barman, who is responsible for keeping the bar stocked.
6. House, who is responsible for furniture and for any special events or parties in the mess.

Despite it being a democracy, the commanding officer (CO) of the unit has right of veto over the mess, and any large changes or events must have his approval. If reasonable requests are rejected then it is considered an abuse of power and can be appealed (except in battlefield conditions). Because of this, the CO is always allowed into the mess, but it is often considered an abuse of power, unbecoming conduct or disturbing the order for a CO to drink in a lower rank mess, except when invited on special occasions.

The officers' mess in a Navy ship or base is called the Wardroom.

Mess dress is the military term for the formal evening dress worn in the mess or at other formal occasions. It is also known as mess kit. Mess dress would be worn at occasions requiring white tie or black tie as the dress.

American usage

In the U.S. Army, the mess is called a mess hall. The Officer's Club is somewhat comparable to the Officer's Mess.