It used to be that radicchio was one of those oddball vegetables that you could find in the grocery store but not very often at farmers markets. That's because early on everyone except for a couple of big farmers had a difficult time growing it. Now, with a better choice of seeds and better knowledge of growing practices, radicchio is becoming more available.
There are many varieties of radicchio. Most of what we see at farmers markets is Treviso, which comes in a red, round head. It is mildly bitter and makes a good salad ingredient. Occasionally, we'll also see the longer Precoce, which has a little more edge and is better cooked.
Because radicchio requires cool weather, it is reaching the end of its farmers market season (during the warm months, commercial varieties are grown in Salinas -- too long a trek for most market growers to make).
How to choose: Look for firm, even hard, heads with vivid colors. If there are signs of darkening, that's usually an indicator that the radicchio has been stored too long.
How to store: Keep radicchio refrigerated in a tightly sealed plastic bag. It'll last at least a week.
How to prepare: Round heads can be torn and used the same as any other salad green. (Radicchio is especially nice finished with some shaved Parmigiano-Reggiano.) The longer heads tend to be more bitter, but they turn almost caramel sweet when grilled until soft and dressed with olive oil and vinegar.