The Shroobs are a race of Poison Mushroom-like aliens that invaded in the Mushroom Kingdom when Mario and Luigi were babies. These events take place in the game Mario & Luigi: Partners in Time. They are a purple, mushroom-like species ruled by Princess Shroob and her older sister. They are from the Shroob Planet and most Shroobs carry ray guns with them. Unlike the similar yet much more benevolent Toads, who are seemingly more human than mushroom, Shroobs seem to have retained most of their fungus ancestry.
They possess very advanced technology, such as ray guns and a massive fleet of flying saucers, powered by the vim of Toads, which is collected by capturing the Toads and draining the vim through the Shroobs' Tree Network, all leading to the Vim Factory. The saucers are capable of shooting energy bolts that turn people into Shroob Mushrooms, as well as causing devastating damage to buildings. The Shroob military is very complex in their processes. They come in different colors and have different attacks. Unlike the Koopa Troop, the Shroobs have everything laid out in advance so that they could both protect vital operations and invade enemy regions at the same time, suggesting they are very intelligent.
The shroobs look very identical to the Toads, except they are purple with white spots at the top, have very stubby bodies and limbs, have pincers for hands, gaping mouths with a pair of fangs, and black irises with red pupils.
 Mario & Luigi: Partners in Time
The Shroobs first started their invasion by destroying Toad Town and capturing many Toads. These Toads were taken to Toadwood Forest, where their vim was drained by the forest's genetically-altered trees. The vim was then collected at the Vim Factory, where Dr. Shroobs gave the vim to Swiggler, a terrifying, bio-engineered Wiggler. While guarding a Cobalt Star shard, Swiggler also drinks the vim and powers the Shroob UFOs which he will use to attack Mario and Luigi.
While attacking Toad Town, they also attacked Princess Peach's Castle in hopes of capturing Baby Peach, but she had left by the time they got in the castle, and instead found the Princess Peach of the future, who had traveled back in time using E. Gadd's Time Machine. They also took Toadiko to Toadwood Forest, and, oddly enough, put Toadbert inside of Yoob. Princess Peach, however, did manage to trap Elder Princess Shroob in the Colbalt Star and then shattered the shards around the kingdom before getting captured by the younger Princess Shroob.
Soon after, they started the creation of the mechanical Yoob and attacked Yoshi's Island. Yoob would eat any Yoshi it could find. These Yoshis would then most likely be converted into more monstrous Yoobs and be used to attack the Mushroom Kingdom, preventing the Yoshis from helping the kingdom in the process. However, Mario, Luigi, Baby Mario, and Baby Luigi went to go stop it, being eaten in the process after the Shroobs had used their Shroob UFOs to make Yoob grow to monstrous proportions.
The Shroobs later commandeered the Koopaseum, using it as a stadium in order to celebrate the conquest of the Mushroom Kingdom. On stage, Princess Shroob fed Princess Peach to the monstrous Petey Piranha. Mario and Company gave chase to the large Piranha Plant under the museum.
Princess Shroob had disguised herself as Peach to take over the Mushroom Kingdom of the future and thus make sure her plan would succeed. She managed to fool everyone in the Mushroom Kingdom until Bowser came to kidnap Princess Peach again, taking Princess Shroob with him to Thwomp Volcano. After being defeated, though, the Shroob Mothership came and took back Princess Shroob.
The Shroobs were apparently melted away when the brothers and Princess Peach discovered the Shroobs' weakness (babies' tears), although it was possible that they simply left the Mushroom Kingdom to look for another place to call home. However, the Koopa Troop seems to have managed to capture various Shroobs and kept them in cold storage, forgetting about them.
Despite regular Shroobs being the main kind of Shroob seen in Mario & Luigi: Partners in Time, only three are fought in the entire game (early on, in Hollijolli Village).
 Mario & Luigi: Bowser's Inside Story
In the lobby of Bowser's Castle, as a hidden Easter Egg, just before Bowser fights Midbus, a Shroob can be seen in the seat on the lower right-hand corner. If Bowser tries to speak to it, it stares blankly and says "...".
Much later in the game, a group of three Shroobs appears as an optional boss in Mario & Luigi: Bowser's Inside Story. They are found in the cold room in Bowser's Castle which needs to have the password of it. The password of the cold room is □, □, ○, △, △, △.
They have two attacks that were very similar to the ones (especially the Guardian Shroobs) they used in Mario & Luigi: Partners in Time. For the first, the attacker will flag down a companion, and they will each shoot a laser at a Mario brother. The other attack is a simple laser blast, delivered to only one brother. Before executing this attack, the attacker will always pirouette, unlike in Mario & Luigi: Partners in Time, in which they would pirouette only before attacking Luigi, and spark before attacking Mario. So instead, the player must see the way it's spinning. It spins clockwise to attack Mario, and counterclockwise to attack Luigi.
After defeating them, Mario and Luigi receive all ten Attack Parts for Magic Window. They may also receive the Shroob Boots, which cause jumped-on enemies to suffer a 50% reduction in attack. Various other Shroobs are visible in the background of the battle, including broken Shroids, Junior Shrooboids, Shrooba Divers, Commander Shroobs and even Elder Princess Shroob. Also, if one uses the Jump Helmet attack and Mario and Luigi walk back in order for the attack to take effect, a single RC Shroober is seen.
The secret battle with the Shroobs is accompanied by several recaps from Mario & Luigi: Partners in Time as Mario and Luigi explain their history to Starlow. After hearing that it was Luigi who ultimately trumped the Shroobs, Starlow sarcastically remarks that he did so by "ferociously crying," referencing the baby tears that were ultimately the aliens' downfall.
It is presumed that the three Shroobs fought, the Shroob in the audience, and Junior Shrooboid are the only survivors of the ending of Partners in Time. Although Elder Princess Shroob and multiple Commander Shroobs and Junior Shrooboids appear frozen in the background of the battle, they may only be a part of the battle comparible to the audience in Paper Mario: The Thousand-Year Door, as Junior Shrooboid is the only other Shroob in the room outside the battle.
 Super Smash Bros. Brawl
The Shroob language as it appears in the text bubbles in the game is just a bunch of nonsensical small picture text. Only one phrase, which translates to "Destroy!", is repeated throughout the game. When the Shroobs talk in the game, they make gibberish sounds. However, they do sometimes use the Latin alphabet, as seen in the battle against Shrooboid Brat. When the Shrooboid Brat was about to attack, the Shroob crowd raised up signs that said either "M" or "L", signifying who to attack. Also, whenever a Shroid uses its iron ball attack, it has the letter M on it for Mario, or L on it for Luigi, although it is vice versa when the Shroid has sparks flying out from its body. A similar thing goes for Elder Shrooboid, when he uses the crystal attack. The Elder Shroob Princess can also speak English. Later on, the language is shown to players using subtitles in parentheses. Whether or not this means that Mario and Luigi begin to understand the language or is just for purposes of clarification is unknown.
 The Shroobs
 Shroob Leadership
 Shroob Monsters
 Shroobified Creatures
 Colonies, Settlements, and Structures
 Game Appearances
 Names in Other Languages