Shiitake are also known as the brown Chinese mushroom. These are the most widely used mushroom in Asia, and are grown in China and Japan and in California on the wood of dead deciduous trees. It gets its name from the Shii tree but also grows on oak and hornbeam trees. The spawn is planted in holes drilled into the tree and crops produce from 3 to 6 years. They are used fresh and dried. To use the dried Shiitake soak in hot water till softened, then snip off the tough stem with a sharp scissors. 

Shiitake are low in sodium, and high in potassium, and are low in calories, and good for heart smart eating. One cup has only 20 calories. They are also loaded with a cholesterol fighter.

For all types of mushrooms, look for clean caps, free from blemishes, slimy spots or signs of decay. Allow for 1/4 to 1/2 pound per person per serving when used as a vegetable.

Store mushrooms, unwashed in a plastic or paper bags. Keep away from strawberries, and other vegetables in the produce refrigerator, as the spores can hasten the destruction of these fruits and vegetables.

Wipe with a damp cloth before use. Do not soak mushrooms.

Mushrooms can be steamed, broiled, baked, fried, or grilled.

Here are some great resources for all things mushroom:
http://www.mushroomcouncil.com/varieties/portobello.html
http://www.melissas.com/rtf/drportmu.htm
http://www.americanmushroominst.org/