Mario Cap

Mario's Cap is an important part of Mario's clothing. The hat is red and has Mario's "M" symbol. This hat has appeared in every game featuring Mario (with the exception of Dr. Mario and Dr. Mario 64, where Dr. Mario donned a doctor's cap instead). Mario has worn the hat since he was a baby.

The Hat's Importance
Mario was given his hat in his first appearance (Donkey Kong) because of the limited advances in video game graphics when the game was made. Since it was difficult to draw hair on a moving character, Mario was given a red cap.

The hat has appeared with Mario ever since then, but was unimportant until Super Mario 64. This was the first game in which Mario could lose his hat. When the hat was lost (by having it stolen by a Ukiki or blown off by a gust of wind), Mario would take more damage when attacked. The same happened in Mario's next platformer, Super Mario Sunshine, but with the added effect of gradually losing health.

In the remake of Super Mario 64, Super Mario 64 DS, a Toad revealed that the cap that Mario wears is special and if were to get blown off of Mario, he would have nothing but bad luck. As hinted in Super Mario 64 DS, Mario's hat may be a source of his strength.

A Useful Vestment
In Super Mario 64, Mario has four different types of hats: the Wing Cap (which enables Mario to fly), the Metal Cap (which enables Mario to walk underwater and to walk through fire), the Vanish Cap (which enables Mario to walk through certain walls) and his regular hat. While these powers were dispersed amongst Mario and his companions in Super Mario 64 DS, Mario's Hat gained a new ability: when worn it could turn Luigi, Yoshi or Wario into Mario.

Additionally, due to certain glitches in Super Mario 64, Mario could end up with more than one hat, and could use the duplicate as an effective weapon.

Alternate Hat Colors
A few Badges in Paper Mario: The Thousand-Year Door can change his hat as well as his shirt. This can be done by equipping the L Emblem Badge (which gives him Luigi's clothes), the W Emblem Badge (which gives him Wario's clothes), or a combination of both (which gives him Waluigi's clothes). Both use 0 BP and have no effects other than changing his appearance. Additionally, in the Mario & Luigi series, the M (or L, for Luigi) on the hat is black, whereas in most games, the M / L is red / green respectively. In Mario & Luigi Partners in Time, this also applied to their baby forms.

Importance in Other Media
In the Club Nintendo comic "Super Mario: Verloren in der Zeit", the DinoBurgers salesclerk takes Mario's hat in exchange for a burger. However, in the second part of the comic, he is seen wearing the cap again.

Trivia

 * In Luigi's Mansion, after Luigi scans the washing machine, it states that Luigi handwashes his hat while Mario just tosses his in the washer.
 * Also in Luigi's Mansion, Mario's hat is one of the many collectibles belonging to Mario. This game makes it seem as if Mario has two hats, as he wears his throughout the entire game (even while Luigi or Madame Clairvoya is in possession of the collectible hat).
 * A giant Mario hat appears on the roof of his house in Dance Dance Revolution: Mario Mix.