It is a thick paste produced by fermenting soybeans with salt and kōji (the fungus Aspergillus oryzae), and sometimes rice, barley, seaweed, or other ingredients.

Miso is high in protein and rich in vitamins and minerals, and it played an important nutritional role in feudal Japan.

In the Kamakura period (1185–1333), a common meal was made up of a bowl of rice, some dried fish, a serving of miso, and a fresh vegetable.

In the Muromachi era, Buddhist monks discovered that soybeans could be ground into a paste, spawning new methods using miso to flavor other foods.

In recent years, many new types of miso have appeared, including ones with added soup stocks or calcium, made with beans other than soy, or having reduced salt for health, among other varieties available.

The ingredients used to produce miso may include any mix of soybeans, barley, rice, buckwheat, millet, rye, wheat, hemp seed, and cycad, among others.

Depending on the microorganism in combination with the variety of soybean or cereal used, many flavor compounds are produced that give rise to the different types of miso.

Miso, depending on the variety, consists of a starter culture called kōji (麹), soybeans, and usually a grain (either rice, barley, or rye).

This mold culture comes from dried A. oryzae spores called tane-kōji (種麹, たねこうじ) or "starter koji" and is isolated from plant matter (usually rice) and cultivated.

[15] In the past, the natural presence of A. oryzae spores was relied upon to create koji, but because of the difficulty of producing the culture, tane-kōji is added almost exclusively in both industrial and traditional production of miso.

Tane-kōji is produced much in the same way as koji, but also has a small portion of wood ash added to the mixture[16] which gives important nutrients to the fungus as well as promoting sporulation.

A. oryzae is an aerobic fungus and is the most active fermenting agent in koji[14] as it produces amylolytic, and proteolytic enzymes which are essential to creating the final miso product.

Temperature, humidity, and oxygen content are all important factors in maximizing mold growth and enzyme production and preventing other harmful bacteria from producing.

Generally, such dishes have the title miso prefixed to their name (for example, miso-udon) and have a heavier, earthier flavor and aroma than other Japanese soups that are not miso-based.

[22] Miso is relatively high in salt which could contribute to increased blood pressure in the small percentage of the population with sodium-sensitive prehypertension or hypertension.

Several studies using salt-sensitive hypertensive models and analyzing long-term intake have suggested that miso lessens salt's effects on blood pressure.