Japanese superheroes usually do not have their powers all the time, but have discrete powered and normal "forms" which they must actively switch between. While Western media also has several such heroes, including Captain Marvel (probably the archetype), Thor, and various incarnations of Wonder Woman, it is far more common for Western heroes to have their powers at all times and merely change into their costume for the sake of appearances (although in some cases, as with Iron Man, the powers are in the costume). In Japanese media, this tendency is reversed, and the need to transform is nearly universal.
Often, the only outward transformation is a change of costume, although a change of size may also be involved, as with Ultraman. Heroes who use armor will usually have it materialize around their body.
Characters with a henshin ability can be divided into two broad groups. For some, the ability to transform is innate. It may be a natural ability of their species, the result of biological manipulation, or a magic spell of transformation. Takeshi Hongo (the original Kamen Rider), Guyvers and Zoanoids from the Guyver franchise, and most magical girls fall into this category.
Others receive their transformation ability from special equipment, frequently super-advanced technology or possessing mystical qualities. Sometimes it only works for a certain individuals, while in other cases, whomever possesses it can transform. The armored heroes from both Saint Seiya and Yoroiden Samurai Troopers fall into this category, as do many Heisei era Kamen Riders.
Whether the transformation is an ability or equipment, henshin heroes frequently use a device of some kind to transform into their heroic form. Such devices are often voice activated, and saying one's henshin call is necessary to transform. This is frequently the case when the hero's powers are magical: the henshin call is actually an incantation or spell.
The form of the henshin device varies from series to series. The Super Sentai series frequently uses wrist-worn devices, while the henshin belt is a trademark of Kamen Rider series. A magical girl's henshin device is typically a piece of jewelry or magical implement such as a wand or staff. Handheld devices (such as cellphones) are also common. Weapons also sometimes double as henshin devices. Some characters with a henshin ability do not use devices.
The hero's transformation is usually shown in a segment of stock footage which is inserted into each episode. These sequences are used to reduce costs–since it only has to be shot or animated once–and to pad out the length of an episode. Some programs which feature henshin do not use stock footage transformation scenes, and even those that do use the stock footage will sometimes omit it. Instead the character will transform in a flash of light or other instantaneous visual effect. Some of the fandom mocks the lengthiness of the sequence, usually pointing out that the enemy should attack the hero. However, there are some henshins with defensive capabilities preventing any attack from harming them while transforming. Other times, it can be considered a form of suspension of reality showing as if it were the first henshin, even if that would not be possible, such as the person changing while falling off a cliff (this happened to Takeshi Hongo/Kamen Rider 1 his first time around).
Most modern Kamen Rider series (from around 2000 onwards), however, now utilize computer animation to dispose of stock transformation sequences in favor of live, brand-new shots every time–in this way, different camera angles, character clothing, and backgrounds can be utilized in every episode to provide a more convincing feeling of transformation for the audience, but retain the same style and effects each time; Kamen Rider 555 was the first to have henshin sequences where the camera could move around as the character was transforming. However, Super Sentai has yet to follow this trend, preferring to stay with stock-footage transformations, usually relying on the characters wearing the same clothes during each episode or a change of clothes prior to transforming.
Capcom's character, Viewtiful Joe, shouts the phrase, "Henshin a go-go, baby!" when he transforms into his comic book style alter-ego. Viewtiful Joe is very much inspired by the Kamen Rider series.