Once, there was a straightforward term for them — picky eaters. These days they go by many different names — Atkins-compliant, lactose-intolerant, wheat-allergic — but they all add up to one thing: Dinner guests who will no longer eat whatever is placed in front of them. Compliments have been replaced by complaints, and those food restrictions and phobias can spoil all the dinner-party fun. "Diets and allergies are without a doubt one of the most boring subjects of conversation ever to attack our social progress," says etiquette expert Letitia Baldrige, former social secretary to Jacqueline Kennedy.

That said, a good host should be prepared. Rule of thumb: Preempt a trip to the ER by asking your guests when they R.S.V.P. if they are allergic to anything. "That way you've covered your bases," says Caroline Tiger, author of How to Behave ($13, www.amazon.com), "but you won't give them license to regale you with all their likes and dislikes."

The guiding principle for guests, according to manners expert Peggy Post, is to be neither a prima donna nor a martyr. If you are served something you can't eat, simply don't eat it (and don't say anything about it). Load up on something else. If your host notices your refusal, turn down the offending food with aplomb and grace, advises Tiger: "Say you can't possibly take any black-bean salad, because the chicken is just too delicious, and you want to save your appetite for that."

If, however, you are one of only a few guests and your abstinence would be conspicuous and potentially embarrassing to you and your host, you should address your allergy or dietary restriction in advance. (You are excused if you would in fact keel over after eating lobster thermidor or lose your good standing with God by partaking of the pulled pork.) Mention your concern casually when you R.S.V.P. and, if appropriate, offer in the same breath to pitch in by bringing something you (and others) can eat. "This brief moment of awkwardness is far preferable to taking the host by surprise the night of the dinner party, when he or she has little recourse," says Tiger.