Super Saiyans.

Regular Super Saiyan Form requirements:

• The Saiyan must have a pure heart. We are told later by Vegeta that the heart can be either pure good or evil.
• The Saiyan must have a sufficient amount of power.
Most of the Saiyan's that transform into this form do so while their emotions very high. Chibi Goten and Trunks are able to transform without extreme emotions, like it came natural to them. A certain nervousness is felt while in the Super Saiyan Form, but, as shown in the Furiza Saga by Goku, once a Saiyan reaches this level, a new awakening opens up in their brain and they immediately understand what's going on. Along with a huge boost in power and speed from their regular form, the Saiyan's eyesight is improved and reaction time is improved as well.

Super Saiyan Second Grade requirements:

• The Saiyan must undergo extreme concentration training.
Although the Saiyan must have already been able to transform into Super Saiyan, no additional training is required for this form. However, the Saiyan must concentrate and basically 'find' the ability to push the Super Saiyan form into this stage from in their spirit. This form is much more powerful than regular Super Saiyan, but because of the semi-bulky muscles, speed actually decreases. Little else is known about this form because the Saiyan's eventually realize that it's futile.

Super Saiyan Full Power requirements:

• Extensive training while in the Super Saiyan Form.
• Substitute regular form with Super Saiyan form until body is used to it.
In preperation for fighting Cell, Goku devises a training plan to max out the Super Saiyan form. His plan was to always use the Super Saiyan form, therefore rendering the body used to it's hardships and making it easier to push power into it. This form could also be Second Grade without the bulk, but it never states this in the Manga or Anime. The reason I believe this form is more powerful than Second Grade is that Goku surpassed Vegeta with this form, and Vegeta was using Second Grade. Being able to reach this peak of masterery in the Super Saiyan form is only accomplished by Goku and Gohan.

Super Saiyan Third Grade (Ultra Super Saiyan) requirements:

• Like Second Grade, the Saiyan must undergo extreme concentration training.
This form is huge, bulky, and very slow. Although the power from this form is nearly equal with Super Saiyan 2, the speed decrease makes this form very useless. Trunks attempts to fight Cell while in Ultra Super Saiyan form, and because of his slow speed, he is beat very badly. After that, the form isn't used again in the Manga. However, it makes an appearance in the Brolli movies, as Brolli uses the form a lot. While in this form, Brolli was still very powerful, but didn't slow down. This made him a very formidable foe.

Super Saiyan 2:

This Super Saiyan beyond Super Saiyan form was first reached by Gohan during the Cell Saga. This form was about equal in power to Ultra Super Saiyan, but the increase instead of decrease in speed makes this form a much more formidable one. The requirements to reach this form are listed below:
• A sufficient amount of training in the Regular Saiyan form.
• Extreme rage and emotions.
• A sufficient regular form power level.
This form is reached basically the same way as the Regular Saiyan Form except that you have to basically max out the Super Saiyan form and go into extreme emotional fits. The only Saiyan we see reach this form for this first time is Gohan, and it is assumed that Goku and Vegeta trained very hard to reach it also. One of the best 'short' battles fought in Dragon Ball Z is between Super Saiyan 2 Goku and Super Saiyan 2 Vegeta.

Super Saiyan 3:

During the Buu Saga, Goku attempts to stall Buu and Babidi by explaining the Saiyan forms. To much surprise, he transforms to a form stronger than Super Saiyan 2. This form is much more powerful than the Super Saiyan beyond Super Saiyan form, and the physical features of the Saiyan are altered quite a bit more in this form. There are only a few known requirements of this form, and they are listed below:
• Very extensive training in both physical and mental abilities.
• The Saiyan must have already used and been used to the normal Super Saiyan form.
• A very high normal form power level.
While in this form, the Saiyan's hair goes very long and thick, the eyebrow's dissapear, and according to Gotenks, the Saiyan's face looks like a 'villain'. Only two beings reach this state in the Series, Goku and Gotenks. The first time that Goku uses this form on Earth, Kaio-sama claims that it uses an unworldy power, and the Saiyan can't hold it very long. Power and Speed are multiplied several times, making this a very formidable form.

Super Saiyan 4:

First discovered in GT by Goku, this form far surpasses all the others in speed, power, and endurance. While in this form, the Saiyan becomes near god-like, having hardly any equals. Super Saiyan 4 has evident changes from the normal Saiyan form, but in appearance, only one change remains constant throughout: red monkey-like fur covering the upper torso, not including the chest, abs, or face and neck. Although both Vegeta and Goku had black hair while in Super Saiyan 4, Gogeta had red. The following is a list of the requirements a Saiyan must meet:
• Must have a tail
• A very large power level
• Reach and learn how to control Golden Oozaru

Mystic Saiyan:

After Gohan pulled out the Zed sword in the afterlife and started training with it, he started becoming stronger because of the impossible weight of the sword. After Goku arrived on the Kaioshin Kai where Gohan was training, he sugested that they test the sword's strength on something. He pulled out a boulder from the ground, but East Kaioshin thought they should test it on something stronger, and created a cube made of the strongest material in the universe. After trying to chop the cube, the Zed sword broke, releasing Rou Dai Kaioshin, a Kaioshin from 15 generations back. This Kaioshin had the power to make any warrior stronger past his limits through psychic ability. The following is the requirements to become a Mystic Saiyan:
• Undergo a 5 hour ritual with Rou Dai Kaioshin running around you singing.
• The user must sit idle for 20 hours while Rou Dai Kaioshin rearranges their ki through psychic power.

The term Mystic is not actually a Saiyan form, but since Gohan is the only one in the series to undergo this process, it is considered a Saiyan power. The Saiyan would power up into Mystic by a form similar to Super Saiyan, and their power and speed is boosted to equal or greater to that of Super Saiyan level 3. The Saiyan's appearance does not change while in this form.

INSTANT TRANSMISSION