I've seen better sides of beef been run over by a combine.
— The Engineer on unappealing sets of items
Item sets are a feature introduced in the Mann-Conomy Update. Depending on the set, new attributes may apply to the wearer when all the items in a set are equipped, altering the role of the class slightly. Examples include increased damage resistance, an immunity to headshots, and silenced cloaking noises. There are currently thirty item sets; the five Polycount Pack contest winners, three Australian Christmas sets, eight Über Update sets, Dr. Grordbort's Victory Pack set, the Swashbuckler's Swag set, nine Very Scary Halloween Special sets, three Australian Christmas 2011 sets, and the hidden Eliminating The Impossible and Dual-Purpose Fruit sets.
The Polycount Pack item sets require players to have the set headwear equipped to activate the item set bonus. The Australian Christmas item sets do not require the corresponding headwear to activate the set bonus. The Über Update, Dr. Grordbort's Victory Pack, Swashbuckler's Swag, Eliminating the Impossible and Dual-Purpose Fruit sets, while they require their respective headwear and, if applicable, Miscellaneous item to complete the set, do not bestow a set bonus. All set headwear and misc. items can be crafted using four Refined Metal and a weapon from the set the desired headwear/misc. that the item belongs to (for example, 4 Refined Metal and a Soda Popper would make a Bonk Boy).
If a part of an item set is equipped, a list of items in the set as well as which are currently equipped appears when the item is inspected, either in the loadout menu or by other players. Unlike the others, the list for the Eliminating the Impossible and Dual-Purpose Fruit sets remain invisible until all items are worn together. If the entire set is equipped, any effects it provides are also listed.
September 30, 2010 Patch
- Added "The Special Delivery", "The Tank Buster", "The Gas Jockey's Gear", "The Croc-o-Style Kit", and "The Saharan Spy" item sets.
December 17, 2010 Patch (Australian Christmas)
- Added "The Medi-eval Medic", "The Hibernating Bear", and "The Expert's Ordnance" item sets.
February 14, 2011 Patch
- Fixed players getting set bonuses applied on tournament servers when the server is restricting the player's weapons using the item_whitelist.
- [Undocumented] Fixed players getting set bonuses applied on Medieval maps for items that are not allowed in medieval mode.
June 23, 2011 Patch (Über Update)
- Added "Black Market Business", "The Man of Honor", "Lawrence of Australia", "One Thousand and One Demoknights", "The #1 Fan", "The General's Formals", "The Airborne Armaments", and "The Clinical Trial" item sets.
July 20, 2011 Patch (Dr. Grordbort's Victory Pack Update)
- Added the "Dr. Grordbort's Victory Pack" item set.
October 13, 2011 Patch (Manniversary Update & Sale)
- Added the "Swashbuckler's Swag" item set.
October 27, 2011 Patch (Very Scary Halloween Special)
- Added "The Curse-A-Nature", "The Tin Soldier", "The Infernal Imp", "The Highland Hound", "The FrankenHeavy", "The Brundle Bundle", "The Mad Doktor", "The Camper Van Helsing" and "The Invisible Rogue" item sets.
November 2, 2011 Patch
- Added a taunt for the Tin Soldier set
November 22, 2011 Patch
- Added a taunt for the FrankenHeavy set
- Fixed set bonuses activating outside of the proper holiday for holiday restricted items
December 15, 2011 Patch (Australian Christmas 2011)
- Added "Santa's Little Accomplice", "Dr. Grordbort's Moonman Pack" and "Dr. Grordbort's Brainiac Pack" item sets.
March 8, 2012 Patch
- [Undocumented] Added the Eliminating The Impossible set.
May 3, 2012 Patch
- [Undocumented] Added the Dual-Purpose Fruit set.