Tom Yum soup originated in Thailand and is sometimes called Tom Yam. It is a soup that normally includes shrimp in a hot and sour serving. The words Tom Yum mean “boiling” or “hot” and “spicy and sour” which perfectly describes the soup. As a staple Thai dish, the famous Tom Yum Goong is served in Thai Square.

With Tom Yum soup, the broth of the soup is made from a combination of fresh and stock ingredients which usually includes kaffir lime leaves, lime juice, fish sauce, galangal, crushed peppers, and lemongrass. Chefs also tell us that the paste for Tom Yum is created by putting all the ingredients together, crushing it, and then stir-frying in oil. Different seasonings are added to accordingly adjust the soup's taste. Normally shrimps are added as a main ingredients. But it will be substituted for chicken, pork, and even beef sometimes.

Over many decades, Tom Yum soup has become a favorite in Thailand and has spread quickly around the world thanks to its relatively simple preparation and fantastic flavors. Many western cultures have also been exposed to Tom Yum Goong and it has become a staple dish served by chefs all over the world. New ingredients have been added here and there, most notably the choices of meat and the herbs used to create the paste. Some modern versions of Tom Yum Goong will use straw mushrooms or oyster mushrooms. Thai chilli pasta (Nam Phrik Phao) which gives Tom Yum soup an orange color and enhances its chilli flavor is included as well.

More Info: en.wikipedia.org