Item synthesis is a part of the gameplay found in the Kingdom Hearts series that allows Sora and/or Roxas to make powerful items, armor, weapons, and accessories using materials collected throughout the various Worlds. Many rare items, including Sora's most powerful weapon, the Ultima Weapon Keyblade, can only be obtained through synthesis.
In all games, item synthesis is exclusively the realm of Moogles, and so can only be accessed through them.
Once the option in the game is opened, Sora must have collected the needed items for the Moogles to make new things out of. Most synthesis materials are dropped by the Heartless and Nobodies, and but occasionally rarer materials, such as Orichalcums, are found in treasure chests instead. After synthesis can first be accessed, only a few items will be available for the Moogles to make. By creating more items, however, the Moogles (and Sora) gain access to lists of items that are more powerful, but also require even rarer materials to make. While there are some changes in how synthesis works in each game, the basic principles remain the same.
 Kingdom Hearts
In Kingdom Hearts, item synthesis is made available after Sora gains the Green Trinity in Agrabah, as he needs it to reach the opening in the ceiling of Cid's Accessory Shop that leads to the Moogle's shop. Twenty-five items are available for creation, and can be made as many times as desired as long as the required materials are available; Ultima Weapon is the only exception as it can be synthesized only once regardless of the availability of the required materials. A "Mix" will appear beside items that have been successfully created to help keep track of the items that have been made and those that have not. After all the items (all the 25 items) have been synthesized at least once, Sora will officially inherit the title of "Master Synthesist" from one of the three moogles in the workshop (the one in the corner and closest to the entrances to the workshop).
New lists of potential items are unlocked by synthesizing a certain number of items from the available options. Lists I-IV each have six items on them, where as List V only has Ultima Weapon.
 Kingdom Hearts Final Mix
While synthesis works exactly the same way in Kingdom Hearts Final Mix as it did in the original game, there are quite a few minor changes. Most notably, new synthesis materials have been added, new items exist on the lists, and the items on each list have been changed.
 Kingdom Hearts II
Kingdom Hearts II has synthesis available nearly from the beginning of the game; usually as soon as Sora and his party come across the first Moogle shop in Hollow Bastion, the qualification of collecting five unique materials to open synthesis has been met through gameplay. Unlike the items for sale in each store, the Moogle shops of other Worlds are linked to each other for synthesis, and so any shop can be used at any time without losing progress from one World to the next.
However, the process of unlocking and creating new items is structured quite differently than it is in the first game. New items are no longer gained by making a certain number of lesser items, but can instead be obtained through Recipes, found throughout the Worlds, as well as through a category called Free Development. By collecting more unique materials and improving the Moogle's ranking, Sora will unlock further items to create. Having five types of materials will open Free Development and synthesis, while gaining 20 types of items unlocks more items on that list. Also, obtaining a certain number of items enables them to be purchased at the Moogle Shops.
 Item Ranks
Each item available for synthesis in the game is assigned a rank, which tells roughly how powerful the item being created is and how common the materials required to make it are. Experience granted appears to be independent of rank, however, with a great deal of overlap in the lower levels. Rank C items are the lowest in quality and require the simplest and most common materials, while Rank S items use rarer items and are much more powerful.
 Moogle levels
In addition to gaining the item itself, the Shop Moogle will gain experience for making the item for you. This in turn unlocks new abilities the Moogle can use to further refine, and create items. Most notably, this involves a special class of bonus items a Moogle can use in item creation: the Bright, Energy, and Serenity materials, which can make items synthesized use less materials, gain the Moogle more experience, or create new items entirely.
The ability to cut the cost of creating items of each rank in half can be gained at later levels.
Synthesis is once again available in Kingdom Hearts 358/2 Days thanks to a Organization XIII-affiliated Moogle who hangs out in the Grey Area of the Castle That Never Was. Once Roxas acquires the Novice Rank on Day 26, following Mission 14, the Moogle will be open for business, including synthesis. In order to create a Panel, Roxas must have all the ingredients required, as well as enough Munny to pay the Moogle to make it for him.
As Roxas completes the various Missions given to him throughout the game, he moves up in Rank. Each time he is promoted, the Moogle offers new items for sale and synthesis. Some items are also unlocked when Roxas collects certain materials needed to create them; many of the Gears become available after the appropriate Gear Component is acquired. In addition, the most powerful Rings in the game offered via synthesis are only offered after the game is completed.
Occasionally, other Organization XIII members hanging around the Grey Area will ask Roxas to use synthesis to create items for them, sometimes even giving him the ingredients. Completing these tasks results in small bonus prizes for Roxas, usually rare synthesis materials, but is entirely optional.
 Synthesis upgrades by Rank
 See also