The number of hopeful competitor robots far outnumbered the available slots in every series of Robot Wars (excluding Series 1). All unseeded competitors were required to undergo a screening process known as the Qualifiers, in order for them to appear on the televised show. The robots would perform in front of the Robot Wars staff, giving demonstrations of control and mobility for entry into Series 2 and 3, and battling against other hopefuls for Series 4 onwards.
For Series 2 and 3, robots had to knock a football into a goal, drive over a see-saw, drive around some breeze blocks, drive in a figure-eight around some large springs and then drive onto a piece of wood and spin on the spot.
Robots battled in a smaller version of the Robot Wars Arena, as the one seen on TV was too large to transport. The qualifier arena was painted to look like its televised counterpart, but did not include the hazards or the house robots. Because of the arena's reduced safety capacity, roboteers were told not to flip their opponents out of the arena during their qualifier battles. The roboteers would be able to enter the qualifiers with more than one robot, but would only be able to qualify with one robot, this in particular affecting Team Mouse in Series 6 and 7, however Team Vader and Team Ming were able to enter with two robots each in Series 7.
The qualifiers always used inexalted house robot Shove as a sweeper/cleaner, to push any immobilised/destroyed robots to the arena entry gates. Shove was used because it was more compact, and therefore proved easier to transport than the Refbot.
The Robot Wars Wiki's list of known qualifier battles that took place from Series 4-7 can be found here, the wiki's list of known robots that failed to qualify for each series can be found here. Examples of Series 6 and Series 7 qualifiers can be found here and here.
As so few teams entered Series 1, no qualifiers were held for that series. In fact, there was even a shortage of robots that were able to fight in the first series, as several robots suffered breakdowns etc. leading up to and during filming. As a result, three Stock Robots: The Mouse, W.Y.S.I.W.Y.G. and Grunt were used to make up the numbers.
Enough teams tried entering Series 2 to make having the qualifiers worthwhile. The qualifiers in this series featured a Gauntlet/Assault Course, similar to what they would have to face in the Heats and Semi-Finals, that the robots would have to complete.
Series 2 also saw the first occurrences of actual competitors coming in as reserves, for the robots that originally qualified but were unable to compete. Panda Monium, Wheelosaurus and Griffon were brought in to replace Cassius, Minotaur and Reckless Endangerment respectively. However, Cassius was able to participate in a later heat, and Minotaur and Reckless Endangerment were entered in the Super Heavyweight Championship at the end of the series.
To still allow some of the robots that failed to qualify to compete in the televised show, a Reserve Rumble was held at the end of the series, for five robots that were unused reserves to fight against each other. Despite this, there were still some robots, like RC Bandit and Team Power's Gnasher, that failed to qualify and didn't feature on the televised show at all.
Series 3 saw a considerable leap in the number of robots applying to participate, as 600 robots came to apply to feature on the televised show. It was not decided at the time of qualifiers that Series 3 would have no Gauntlet and Trial, so a Gauntlet/Assault Course was still used as a form of qualifier. As more robots were applying, the judging was much more strict than it was in the previous series. For example, some robots, like the original Major Tom, completed the course, but were still refused entry to the televised show.
Series 3 saw yet more reserves come into play, Binky, Flipper and Steg-O-Saw-Us replacing Daisy, The Parthian Shot and T-Wrecks respectively. There were other reserves, notably Wolverine, that still weren't used.
To still allow some of these robots that failed to qualify to feature on the televised show, a handful competed in some of the side competitions for the Third Wars, such as the Robotic Soccer Championship and the Pinball Warrior Tournament, it was here that robots such as Six Pac and Dominator proved themselves to be worthy robots. However, some robots that did enter the main competition, including Evil Weevil, Razer and Killerhurtz were still used in these side events as well, to compare these other robots to the robots that were fighting in the main event.
A record-breaking "1000" teams applied for this series. Because the format of the show had changed to pure battling, the qualifiers now consisted solely of battles between hopefuls. The battles were head-to-head, with the possibility of draws being declared in the event of no clear winner. Some robots were able to fight more than one battle. Occasionally, seeded robots were invited to take part in qualifiers, although they would qualify regardless of outcome.
Due to some of these battles being decidedly "boring" battles, even robots that won their qualifier battles were refused entry into the televised show, robots that suffered from this included Dantomkia 1.0. Conversely, robots that lost their qualifier battles, like Warhog, were given discretionary places into the main competition, as they put on a more entertaining show.
V-Max, which originally failed to qualify, replaced Onslaught, after the latter withdrew from the main competition in the last minute. There were other reserves for the Fourth Wars, notably Charybdis, that weren't used.
Unlike the Third Wars, no robot that failed to qualify participated in the side events that were held during the filming of the main competition.
The Fourth Wars also saw the first instance since the First Wars of a robot being able to qualify without having to go through the Qualifiers. Scorpion was granted automatic entry, as the Robot Wars Staff decided that it would look good in the arena.
The qualifiers in Series 5 were done in front of a live audience to promote the upcoming series. As a result, Craig Charles was even used as MC, and the house robots were put into the arena for the qualifiers. Seeded robots were once again invited to take part in qualifier battles for their entertainment value, but would not be denied qualification if they lost a battle. Most of the battles were still head-to-head, but melees were often used to judge more robots more quickly.
Winning a battle still didn't guarantee a robot a place in the main competition, robots such as Cataclysmic Variabot and Barbaric Response failed to qualify despite winning their battles. However, some robots that lost their battles, such as Dome, were awarded discretionary places.
For the Sixth Wars, the qualifiers were mostly four-way melees, like that of the first round battles in the televised show, however some would consist of only two or three robots depending on availability. Unlike in past series, each robot was strictly limited to one qualifier battle, and the winners would be guaranteed qualification. However, losers could still be awarded discretionary places.
Notably, champions of the next series, Typhoon 2, would fail to qualify for this series, after performing inadequately in its qualifier battle.
In Series 7, the qualifiers were the same format as those of Series 6 - up to four-way melees with qualification guaranteed to the winner(s), and others being granted discretionary places. Around 200 robots attempted to qualify for the series.
For the first time in Robot Wars, teams from outside the UK attempted to qualify. Twenty-two roboteers from eastern and central Europe pooled their money for transportation to the qualifiers. Arthur Chilcott of Random Violence Technologies even allowed them to use his backyard as a camp-ground. Although some of these failed to qualify, some of those would still fight for the right to enter in the Third World Championships, held at the end of the series.
Oddly, Typhoon 2 lost its second consecutive qualifier, this time to Ewe 2, losing alongside Big Nipper and Araknia, but Typhoon 2 was this time given a discretionary place for the Seventh Wars, as were Big Nipper and Araknia. Typhoon 2 would then go on to win the series.
Once again, Team Mouse were forced to only qualify with one robot, this time opting for Velocirippa, but Mighty Mouse was still listed as a reserve due to the team living nearby to the studio and being able to make it to filming in a short amount of time.
As it turned out, Mighty Mouse was later brought in as a substitute, but there were some other occurrences during the Seventh Wars where reserves could/should have been brought in, but weren't. Terrorhurtz was unable to fight, but was merely classed as "disqualified" without being replaced. Thunderpants also broke down as it entered the arena, but the battle was not restarted.