Why Are Superheroes Traditionally Shown Wearing Underwear-Like Spandex?

Three reasons:

1. The best way for artists to draw muscular figures is to have them wear tights or nothing at all. Superheroes wouldn't look as super if they wore clothes that hid their muscles and would be downright laughable naked (except Wonder Woman).

2. Tights allow the superhero to wear their costume under their clothes for quick changes. This gives the writer the ability to have the superhero believably be on the spot for any crime.

3. When superheroes first came about with Superman, Batman, etc back in the 1930s, circus strongmen and wrestlers were very popular examples of superhuman feats. They often wore tights with the "speedo-looking trunks" on the outside. So, when superheroes were created, this fashion was adapted. You'll notice that most heroes that were created after the popularity of strongmen died have tights but no trunks on the outside like Spider-Man.

Fuzzy Dan

becouse if no one could stop us wouldent we all be wearing colorfull spandex. deep down iside you know you would. and i aswell cant wait to see The Dark Knight.

They wanted something that was easy to move in && they needed something to look hot [for the ladies] in also.

Freedom of movement (like dance wear) and to show off those awesome, huge ... bulging ... muscles!

I think they are trying to boost the 'sex appeal' of these men.

Ha...that's what they do to the poor superheroines at least.

=)