If you don't have tarragon, can't find it in stores or just plain don't like it, there are several good substitutes that you can use in its place.
What is Tarragon?
Tarragon is a perennial herb with grass-like growing habits. There are French and Russian varieties of tarragon, but it's the French variety that cooks favor. It's prized for its skinny leaves, which are highly flavorful and aromatic. French dishes rely heavily on tarragon, and it's one of the herbs that's included in fines herbes. Tarragon is also an essential ingredient in Bearnaise sauce, and for that there's really no substitute. But, when you see it on the ingredient list for delicately-flavored, French-inspired dishes that include eggs, fish, chicken or cheese, you should be able to make a successful substitution.
Flavor-wise, tarragon tastes like licorice or anise. While many people love this flavor, many others dislike it, or would prefer not to experience it in a savory dish. When making a substitution, you should consider whether you want to keep the licorice flavor, or replace it with something else entirely.
It's also important to note that fresh tarragon and dried tarragon taste very different from one another, so substituting one for the other usually isn't your best bet (though, it is an option).
Substitutions If Your Recipe Calls for Fresh Tarragon
Use an equal amount of fresh chervil or fennel fronds. If you don't have either of these fresh herbs on hand, you can also use a pinch of fennel seed or anise seed for each tablespoon of fresh tarragon called for in the recipe. Not sure how much a pinch is? It's somewhere between 1/16 and 1/8 of a teaspoon.
Substitutions If Your Recipe Calls for Dried Tarragon
Use a pinch of fennel seed or anise seed for each teaspoon of dried tarragon called for in the recipe. If you have fines herbes in your spice collection, you can also use it in place of the dried tarragon. Since it includes tarragon as one of its ingredients, this will keep some of that licorice flavor in your recipe.
Substitutions If You Don't Like Tarragon
Use an equal amount of dill, basil or marjoram in its place. These herbs don't have that licorice taste, but should still work well in the types of dishes that tarragon is typically used in. Another option is to simply omit the tarragon. As long as there are other spices to flavor the dish, you probably won't miss it at all.
Looking for a Substitute Because You're Not Sure If You Like Tarragon?
If you have tarragon on hand, consider using half as much as the recipe calls for as an introduction to this new-to-you herb. While tarragon is a common ingredient in French cuisine, it isn't a flavor that Americans are as accustomed to, so you may want to ease into using it in your cooking, particularly if you don't have an adventurous palette. Also know that, unlike other herbs, the flavor of tarragon intensifies considerably when it's heated up. So, that's another good reason to go light in the beginning.
Using a fines herbes blend in place of the tarragon is another way to ease into things. Since fines herbs also contains chervil, chives, thyme and parsley, the tarragon will be layered with flavors that you're already familiar with. A one for one replacement should give you good results.
Can't I Just Swap Dried Tarragon for Fresh?
You can substitute dried tarragon for fresh, but you probably won't be very happy with the results. They taste very different, mostly because fresh tarragon has a far more intense flavor than dried tarragon. But if this seems like the best substitution given the herbs you currently have access to, use one teaspoon of dried tarragon in place of one tablespoon of fresh tarragon.
If you grow tarragon in your garden, consider freezing some of your harvest, so you'll have access to "fresh" tarragon year round. Freezing will preserve the intense flavor of tarragon in a way that drying can't.