heh, i forgot, i come from northwestern china, and people there weregenerally taller than other asians... I think it's due to a mix of blood with the mongols... or something like that... idk.

The primary weapon of the samurai was the bow, and next comes the spear, and the swords were actually the 'last defence'. This held true until kamakura period, when the mounted soldiers used the sword as their primary weapon, for running down enemies. The japanese also like bigger weapons like the naginata, which they used to cut down horseman. in all these cases, the addition of a shield to the samurai gear would be either impossible or inconvienent.

then, around edo period, the samurai went out on foot with no armor at all (I've no idea why, maybe it's just a matter of style or show of manliness?)

im unconvinced by the '2 handed weapons dont work with shields comment,
we have numerous references throughout history of soldiers having shields AND long weapons at the same time
the macedonian phalangite, and the byzantine kontarion pikemen.
both had shield while holding pikes, much in the same way renaissance and japanese pike formations did.

its definately possible through the use of the guige strap and designing the shield the right way..

the byzantines used kite shields smaller than the norman variety we believe but they had guige straps and enarms that allowed one to hold the pike with your left hand, while having the shield rest on your left arm and being supported also by the guige strap,
the macedonians used a very similar system exccept with roundshields.
that said, these blocks of men with large yari, were in an age where the bow didnt quite decline, but was being substituted by musketry and we know that a wood and leather shield is mostly unable to stop bullets.

as for mounted archery, its been also well established that shields were used alongside mounted archery by a variety of peoples , the mongols the turks, the rus, the sassanids and persians before them.

its not that it doesnt work,
i think its probably the reason that they didnt need it
as was noted the japanese had the large square O sode, which were freely swinging lamellar panels attatched to each shoulder that would, hopefully stop arrows coming at you.

also consider japanese society back then, they were at times FIERCELY traditional (case in point the entire edo period)

Far as I've understood it, the Edo period actually established traditions not in effect up until then.

Also, when it comes to horse archery, a shield is very plausible with a bow that isn't as tall as you are (far as I've understood it at least).

I'll admit, Japanese warfare and weaponry is definitely not at all my area of expertise, so if I make some errors please correct me :)

As far as I know the Japanese bows were huge (from what i've seen larger than a longbow), so I think a shield may definitely interfere.

I agree with Jean and Carl's theory that japanese armour was sufficient against slashing and piercing damage, and due to the lack of heavy blunt trauma weapons (I have seen pictures of a heavy club like weapon, but they seem rather rare, and other japanese weaponry as the katana seems to be built lightly more for sharp cutting damage rather than blunt trauma... This view could be completely incorrect and shaped by popular media however :P ) the rigid defence of a shield may have just not been needed.

Just my uneducated 2 cents, I hope there are no glaring flaws.

Here is a quote from a book by Jonathan Clements, A Brief History of the Samurai Page 24 http://books.google.com/books?id=gyqN0PRuoeUC...amp;f=true

Quote:

The design of warrior's armour began to reflect the increased role of mounted archery. The top of the helmet remained the toughest point of the armour suit, and its neck guard flared out even further. This afforded the maximum protection and deflection from arrows fired directly above the wearer's head. While this might seem strange at first, it suggests that when charging at their enemies, Japanese soldiers would do so with their heads low and eyes down, presenting the arrow-deflecting curves of their helmet head-on the the foe. With the need to keep both hands free for using a bow, no shield was possible. Instead Japanese warriors began to favour large, shield-like attachments that formed square pads attached to their shoulders.

During the periods when bows were one of the main weapons, large "sode" (sholder armor) were attached to the armor very loosely by cords tied on top of the armor, this allowed the sode to be moved or removed easily, in later periods when the bow was no longer a primary weapon, the now much smaller sode were firmly attached to the armor with toggles, the sode could not be as easily moved or removed on later armors.

Samurai armor showing the large old style sode (shoulder armor).
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The samurai started to use shields mounted on the shoulders of their armor called "sode", the use of sode allowed the samurai to use both hands while riding a horse, this was a significant innovation. If you look at an image of an early samurai in O-Yoroi armor riding a horse you will see how well the armor covered the vital areas. By not having to hold a shield in their hands the samurai would have had a significant advantage over a house mounted enemy who was holding a shield. Instead of wondering why the samurai did not use hand held shields you might as well wonder why other cultures did not use sode instead of hand held shields while fighting on horse back.

Japanese foot soldiers did at one time use hand shields, Dorothy Perkins notes the use of "wood and leather shields" by foot soldiers as early as the mid 6th century. The Japanese government eventually started to rely quite heavily on horse mounted warriors sometime between the 700s and 800s to provide protection as the conscripted foot soldiers proved to be less than adequate against enemies who increasingly used horses. The battles with the Emishi, who were skilled at riding horses necessitated a similar horse mounted warrior to fight them.

At some point in time the hand held shield seems to have been completely abandoned in Japan, its not as if the Japanese never used hand held shields.

If you look at this image of an early samurai riding a horse you will see how well the O-Yoroi armor protected the vital areas. Note the tachi and yumi.
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Samurai fighting from behind shields.
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Ashigaru using shields
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what about ailettes used by 13th C knights?

that said, eric makes a good point,

espcially for cultures like the mongols and turks which were heavily based on horse archery in the upper echelons.

that said these guys had lots of lancers as well.

i already mentioned the turkish shields aka kalkan. which can be held so that they rest on the upper arm and shoulder,

but O sode covered both shoulders.