Udon is often served hot as a noodle soup in its simplest form, as kake udon, in a mildly flavoured broth called kakejiru, which is made of dashi, soy sauce (shōyu), and mirin. It is usually topped with thinly chopped scallions. Other common toppings include tempura, often prawn or kakiage (a type of mixed tempura fritter), or aburaage, a type of deep-fried tofu pockets seasoned with sugar, mirin, and soy sauce. A thin slice of kamaboko, a halfmoon-shaped fish cake, is often added. Shichimi can be added to taste.
The flavor of broth and topping vary from region to region. Usually, dark brown broth, made from dark soy sauce (koikuchi shōyu), is used in eastern Japan, and light brown broth, made from light soy sauce (usukuchi shōyu), is used in western Japan. This is even noticeable in packaged instant noodles, which are often sold in two different versions for east and west.
There are many stories explaining the origin of udon.
One story says that in AD.1241, Enni, a Rinzaimonk, introduced flour milling technology to Japan. Floured crops were then made into noodles such as udon, soba, and pancakes which were eaten by locals. Milling techniques were spread around the country. In the Edo period, the thicker wheat noodle was generally called udon, and served with a hot broth called nurumugi(温麦). The chilled variety was called hiyamugi(冷麦).
Another story states that in Nara period, a Japanese envoy was introduced to 14 kinds of confection whilst in Tang Dynasty China. One of them was called sakubei(索餅). Sakubei was listed as 'muginawa'(牟義縄) on Shin Senji Kyo(新撰字鏡), a dictionary which was published in Heian Era. The muginawa is believed to be an origin for many kinds of Japanese noodles. However, the muginawa listed on Shin Senji Kyo(新撰字鏡) was made with wheat and rice flour.
Another historical story for udon claims that the original name of the noodle was 'konton', which was made with wheat flour and sweet fillings.
In Chinese, udon noodles are called wūdōngmiàn (烏冬wūdōng or 烏冬麵), and sometimes wūlóngmiàn (烏龍麵), which has the same characters as those used in "Oolong" tea (wūlóngchá, 烏龍茶), though there is no semantic relation between the two usages.
Like many Japanese noodles, udon noodles are served chilled in the summer and hot in the winter. Toppings are chosen to reflect the seasons. Most toppings are added without much cooking, although some are deep-fried. Many of these dishes may also be prepared with soba.
Yaki udon: Stir-fried udon in soy-based sauce, prepared in a similar manner to yakisoba. This originated in Kitakyushu of Fukuoka Prefecture. (Note that while yakiudon is made with udon, yakisoba is not made from buckwheat soba, but with steamed Chinese-style ramen.)
Miso-nikomi udon: hard udon simmered in red miso soup. The soup generally contains chicken, a floating cracked raw egg that is stirred in by the eater, kamaboko, vegetables and tubers. The noodles are extremely firm in order to stand up to the prolonged simmering in the soup; additionally, the noodles do not contain salt, so as to avoid over-salting from the salt in the miso.
Zaru udon: Chilled udon noodles topped with shredded nori and served on a zaru (笊 or ざる), a sieve-like bamboo tray. Accompanied by a chilled dipping sauce, usually a strong mixture of dashi, mirin, and shoyu. Eaten with wasabi or grated ginger.
Bukkake udon: Cold udon served with thick dashi-broth.
Hadaka udon (naked udon 裸うどん): Cold udon served on its own.
Kijoyu udon: Served in a cold soup of raw (unpasteurized) soy sauce and sudachi (a type of citrus) juice, sometimes with a bit of grated daikon.
There are wide variations in both thickness and shape for udon noodles.
Hakata udon (博多うどん): a thick and soft type from the Fukuoka.
Dango-jiru (団子汁): similar to the above Hohtoh, from Ōita Prefecture. Nominally a "dumpling soup", it resembles very thick, flat udon.
Okinawa soba (沖縄そば): also called suba, a regional Okinawan noodle made by adding some vegetal ash to the flour, similar to how ramen is made. However, it is very similar to udon.
There is also a dish called udon in Palau, because of the former Japanese administration. The broth is soy sauce–based like Japanese udon. However, as there were many immigrants from Okinawa, it uses less broth like Okinawa soba. Most notably, the noodle is that of spaghetti, as it is easier to acquire there.