Markets stock a variety of cultivated mushrooms, but many people prefer wild mushrooms, which are often more flavorful. Be careful when picking wild mushrooms (some species are poisonous) and always cook them thoroughly, both to release their flavors and to convert their proteins into a more usable form.
To prepare fresh mushrooms, first trim off the bottoms of the stems, then wipe them off. Don't rinse or soak them, for they'll absorb water and turn mushy when you cook them. Dried mushrooms are often excellent substitutes for fresh, though some species don't dry well. You can reconstitute dried mushrooms by soaking or simmering them. Don't throw out the soaking liquid--it can add more flavor to your sauce than the mushrooms themselves. You can also pulverize dried mushrooms with a food processor or blender, then use the mushroom powder to flavor sauces and stews.
For substitutions for mushrooms in general, click here.