Fresh herbs will almost always be more satisfying than dried. However, dried herbs are often more readily available. If a fresh herb is specifically called for in a recipe, it is usually because the herb is added at the end of the preparation, not cooked with it; and dried herbs should not be substituted, for they will not impart the fresh taste and color desired.
Although many cookbooks tell you to substitute 1 teaspoon of a dried herb for 1 tablespoon of fresh, there is no easy formula for this, because the strength of an herb varies from plant to plant, season to season, and variety to variety. As a general rule, start by using whichever you have—fresh or dried—in the amount given in the recipe. If you’re concerned that you may be adding too much dried, start with a smaller amount. You can always add more.