Announced at E3 2001, Pokémon Stadium (ポケモンスタジアム, Pokémon Sutajiamu), commonly known as Pokémon Stadium 1 to avoid confusion with its Brawl-introduced counterpart, Pokémon Stadium 2, is a stage with a beginning layout that cycles through four other transformations. In All-Star Mode, this stage is played on when the player faces Pikachu and any of its teammates. The stage appears in both Melee and Brawl.
The basic stage is comprised of three platforms; one large platform, and two smaller ones. The large platform is on the bottom is flat with two very small elevations for the sides of the stages. The two small platforms of the basic stage float at the medium between 1/4 and 2/4 for the first platform and 3/4 and 4/4 for the second. The basic stage is physically symmetrical. After a while, which could be from a few seconds to a few minutes, the basic stage will transform into one of the other four stages, as indicated by the jumbo television screen that is showing a program of the match being played. After the change into a different stage, the level always goes back into its original basic stage. The same stage could appear twice in a row, but only after the basic stage interval.
The second stage is a forest/jungle area. It is somewhat flat on the main platform. There is a small elevated plateau on the right, it ends about 1/3 of the level through, followed by a flat grass area that goes the rest of the level through. There is also a river a little left of the halfway mark that has a minor depression. There are three aerial platforms of this second stage. One medium sized one halfway of the staged and risen and another one higher than it and to the left. The second platform starts halfway of the first platform. The third and final platform is to the right that is flat and very small.
The third stage is a water level. It is flat on the right side with an elevated left side that depresses back down before the stage ends at the left side. On the smooth right side of the level, there are two aerial platforms. A lower one just left of the halfway mark, and a higher one to the right of it. A windmill also appears on the left side. The arms of the windmill spin slowly and the player can stand on them, but they will dump them off if the player stands on the current lowest arm. In Melee, the arms of the windmill can be passed through, but in Brawl, they cannot be passed through.
The fourth stage is a fire area. The bottom platform is not very flat at all. The middle is a small valley. Where the valley ends on the right, it is flat through the rest of the level. There is one floating platform that follows the elevated plateau for the rest of the level. That floating platform is flat. To the left of the valley in the middle is an unusual shaped obstacle. As soon as the valley ends on the left, there is a strait up elevation that combatants cannot pass through. Most characters need to double jump this hurdle, which the player can stand on the top of it. After the platform that is the top of the hurdle, it goes straight down again, but only half way. At a 45 degree angle up and left, there is a medium sized platform that sticks out to the end of the level. It is possible to stand under that 45 degree angle and the platform to the left.
The fifth and final stage is a rock/mountain stage. This stage has a very rugged bottom platform, with 5 aerial platforms in the center. Two on the left are very small and attached to the mountain, they are parallel and are flat. The other three on the right are all on top of each other, each with their own different angle. The bottom aerial platform is at a steep angle that touches the ground so that the player can walk on it. The mountain itself is the largest part of the stage. The top can be stepped on and the left side of the mountain is a steep drop to the edge of the stage. There are many opportunities for infinites against the wall and many partial caves of life. The left side of the stage near the ledge has very little ground to stand on.
Pokémon Stadium was one of the few stages that people in tournaments have almost never questioned being legal, and for a long time, was a starter. Many people considered Pokémon Stadium to be one of the fairest stages in Melee because the shifting terrain was generally not very disruptive to gameplay, and for some time, was thought to prevent excessive projectile camping. However, it was found that Fox and Falco had a large advantage on this stage because the geometry of the stage and terrain allowed them to projectile camp with Blaster very safely. The large horizontal size of the stage benefits their movement greatly, while hindering other characters who cannot traverse across the stage as easily, while the low ceiling allows even earlier KOs for Fox's two main finishers. The transformations also benefit Falco, and especially Fox, greatly in several matchups by either giving them more room to approach or safe spots to camp in. As such, it has been deemed a counterpick stage in more recent rulesets, with some Melee players arguing that it should even be banned because the advantages given to those two characters are far too overwhelming for the majority of the cast.
In Brawl, the stage is more debatable in its neutrality; while it's a starter in the Unity Ruleset and always in seven stage starter lists, Lylat Cruise is often chosen over it in five stage starter lists, while never being a starter in three stage starter lists. The change in unquestioned neutrality comes from changes to the stage itself; the lips at the edge are much more thin and deceptive, occasionally flustering recoveries from slightly underneath the stage or even regular recoveries away from the stage. The most notable change in the stage, however, comes from the redesigned water transformation; while it looks almost the same as in Melee, the blades of the windmill are no longer soft platforms and cannot be passed through in any direction, resulting in stage traversal difficulties and significant caves of life while the windmill is present. Brawl players also tend to camp out the Rock and Fire transformations, hiding behind the large rock and the tree until the transformation has ended.
While the stage is not specifically based on anything from the Pokémon video game series, it may be a general representation of a Gym. One reference could possibly be from the game Pokémon Stadium. The arenas in Pokémon Stadium have the Poké Ball logo in the center with crowds watching the battle. While there is a general stage, similar to the standard form of the stage, the field can transform into various stages, including Rock and Water, although they are much different from the rock and water forms represented in the stage. Another source is the Pokémon anime, in which, during tournament battles, the stage changes its terrain frequently to either hinder or help either Pokémon. This was seen at least in the preliminary rounds of the Indigo League (in the anime's second season), in which each of Ash's four initial battles was in a different battlefield representing a specific elemental type.
There are two music tracks for this stage. The primary track first appeared in the title screen of Pokémon Red and Blue. The alternate theme is a mix between wild Pokémon battle theme and the track used when battling a Gym Leader. Both are from Pokémon Gold and Silver. 
In Brawl, the stage also uses the music of Poké Floats.
Trainers come from far and wide to congregate at Pokémon Stadiums, the ultimate venues to show off their Pokémon teams. There are many different arenas with varying terrain designs, some of which afford advantages to certain Pokémon. This particular stadium is reserved for huge events and boasts a big screen and spectacular fireworks.