Bring water to a boil in a pot. Add the galangal, lemongrass and kaffir lime leaves and allow the herbs to impart their scent (this should take about 30 seconds).
If you like your soup spicy, add the bruised chili peppers. Season with fish sauce for saltiness and lime juice for sourness: taste the soup for the correct seasoning, since the sourness of the lime juice varies as well as the saltiness of the fish sauce. Add the lime juice last and a little at a time.
Add the mushrooms and cook in the boiling liquid for one minute. Add the shrimp and turn off the heat. Stir to cook the shrimp. This should only take about a minute since the shrimp cooks very fast.
Spoon into a bowl and sprinkle with picked cilantro leaves. Serves 4