Udon is most often served hot as a noodle soup in a mild, salty broth. Its simplest form is called kake udon. This consists only of dashi stock, soy sauce (shōyu), mirin, and udon noodles. Different things can be put on top of the soup as toppings. Often thinly cut scallions are used, along with other common toppings such as kamaboko, prawn or vegetable tempura, some spices, and wakame.
The flavor of the broth and toppings, as well as even noodle texture and thickness, is different from region to region. Usually, dark brown broth made from dark soy sauce (koikuchi shōyu) is common in eastern Japan. Light brown broth made from light soy sauce (usukuchi shōyu) is common in western Japan. However, many other regional types exist outside of just broth color and mainly vary in toppings used.
- Tempura Udon (topped with tempura such as prawn and squid)
- Tororo Udon (served with grated yam on top)
- Kitsune Udon (topped with deep-fried bean curd)
- Tikara Udon (topped with rice cake)
- Curry Udon (Udon soup is curry,which contains dashi)
- Niku Udon (topped with beef that has been seasoned with mirin and sugar)
- Tanuki Udon (topped with agedama)
- Kamatama Udon (raw egg in boiled udons)
- Bukkake Udon (consists of noodle soup)
- Kamaage Udon (to eat without tighten the boiled noodles)
- Tamagotoji Udon (udon closed with egg)
- Tsukimi Udon (topped with raw egg)
- Nabeyaki Udon (noodles braised in a pan with many toppings)
- Yaki Udon (seasoned baked noodles in a sauce)
- Nabe Udon (to eat with vegetables in a pan)
- Zaru udon (cold udon on zaru)
According to JAS (Japanese Agricultural Standard), to be udon, a round noodle's diameter has to be over 1.7 mm and a flat noodle's width over 1.7 mm. Udon dough is made from wheat flour and salt.