Matchmaking is the process of determining appropriate pairings in Hearthstone.
The matchmaking process functions differently for each type of play. Casual play and Ranked play at Legend rank determine pairings using each player's matchmaking rating (MMR) for that type of play, while Ranked play below Legend rank is determined by each player's rank, and Arena pairings are determined by each player's win/loss record for the current run. In each case the system attempts to find a player of as similar rating, rank or record as possible.
 Matchmaking ratings
Matchmaking rating (MMR) is a special internal rating system used for certain types of matchmaking. Hearthstone uses separate MMRs for different types of play, such as Casual Play mode and Legend-level Ranked play. Each MMR is determined independently and does not affect other types of play. MMRs are 'hidden' and cannot be viewed by players.
MMRs are used for the following types of matchmaking:
MMR is adjusted after each match, depending on the outcome of the match, and the MMR of each player:
- MMR is gained with wins and lost with losses.
- The amount of MMR gained or lost is dependent on the MMR of your opponent.
- Casual Play mode MMR is adjusted quickly when a player experiences a win or lose streak.
All forms of matchmaking function to find a suitable opponent from the currently available pool of players. Pairings are therefore affected not only by each player's rating or rank, but by which other players are currently awaiting matchmaking. For example, different times of day often attract different types of players, with certain times typically featuring a slightly more competitive pool of players. Because of this, each type of ranking is entirely accurate only for that same quality of population. Such variations are not fixed, and may vary by day, time of day, or in relation to calendar or game-related trends.
Because wins and losses will ultimately result in an adjusted rating or rank, such variance can also lead to a yo-yo effect with pairings. For example, a player who normally plays Casual Play mode in the mornings may try playing at another time of day, only to find the competition more fierce. This discrepancy in the player's matchmaking will likely lead to the player losing several games in a row, resulting in the player's MMR dropping rapidly in order to select opponents of equal ability. However, should that player then return to playing in the mornings, they may find the opponents selected for them to be extremely easy to defeat, due to the adjustment of their MMR.
- Zeriyah on Twitter - "Arena matches you against someone with as close of a win/loss record as you have."
- Hearthpwn - Blue Tweets, MLG/ManaGrind Tournament #10 Review, Legends of Hearthstone W9
- Patch 188.8.131.5217
- Ben Brode on Twitter - "Casual tries to quickly find you an opponent with equal skill. It adjusts your hidden MMR quickly when you lose or win streak."
- Beta Forums: Ranks reset, ELO/MMR stays the same - is this working?