Varying from tan to dark brown, Shiitakes, both fresh and dried are almost always used cooked. They are much tougher than Criminis and have a more intense mushroom flavor. Dried they run from 1-1/2 to 2 inches in diameter and weigh 7 to 10 to the ounce. Fresh they run from 1-1/2 to 2-1/2 inches in diameter and weigh 3 to 5 to the ounce. Of the photo specimens the three on the left are fresh and the two on the right are dried.
Dried Shiitakes are generally sold as "dried black mushroom" or "dried forest mushroom". In any case they are easily identifiable due to the texture of the top of the cap. Fresh shiitakes are now produced in large quantity in several areas of the U.S. and are ever easier to find. In many East Asian markets a special grade of dried shiitakes that is very light in color with deep crackling on the top surface is sold at very high prices, often in windowed gift boxes.
More on Mushrooms.
Buying: Dried Shiitakes are now available in just about all Asian and Southeast Asian markets and in most Supermarkets throughout the US. Fresh are available in most markets which carry Asian fresh produce, at least here on the West Coast. Both are best bought from an ethnic Chinese, Korean or Southeast Asian grocery where they will be fresher and much cheaper than from the supers or health food outlets.
Purchased fresh they should be firm and a bit dryish. They are sometimes sold rather damp and soft, but that way they will start to mold in a much shorter time.
Storing: Dried they can be kept for at least a years in a dry, tightly sealed container kept away from sunlight. Fresh they can be kept up to 14 days loosely wrapped and refrigerated. They are much more tolerant of plastic wrapping than white mushrooms are.
Cooking: Chinese recipes almost always call for dried shiitakes and fresh are rarely used. Japanese and Korean recipes very often call for fresh. The stems aren't used either fresh or dry. They're tough and cooking doesn't soften them much.
Soaking Dry: Dry shiitakes should be re-hydrated before using. Better flavor results from soaking in cool water for 1 hour or more, but if you are in a hurry, you can soak in very warm water for about 30 minutes. Then remove the stems and cut or not following the recipe. The soaking liquid can be used in some recipes but will make the mushroom flavor too strong in others. Most recipes presume you will discard it, but some call for it. Use your own best judgement.
Yield: One ounce of dry mushrooms will weigh about 4 ounces soaked and wrung out.
Soaking Fresh: Fresh Shiitakes are generally dryish so stem them, cut or not following the recipe. Soak them in warm water for 20 minutes or so and then wring them out well. This procedure transforms the texture from tough to silky, and they will cook much better. This procedure is especially important when using them to substitute for crimini or white mushrooms where tender texture and a lot of moisture is expected.
Subst: Fresh shiitakes generally cannot substitute for other mushrooms because or their distinctive flavor, but sometimes it can work (see about soaking above). Dry ones would very rarely work. Dried and fresh can be substituted for each other with appropriate adjustment for both weight and flavor. It will change the flavor of the recipe some. Other mushrooms generally cannot be substituted for shiitakes - they are too moist, tender and weak in flavor.