If you love dogs, you face the constant temptation to add one more to the family. While there are many reasons for having a second dog, choosing the right one depends on answering the right questions--ahead of time: Why do you want a second dog? What kind do you like? How are you going to introduce it into your household?

Perhaps the most basic reason to have a second dog is that it may add to the harmony your first dog brings. While a simple desire to enjoy twice the companionship has potential for bliss, it is not without possible complications. If you are trying to merely duplicate the first dog, problems are almost assured. If the second dog does not mirror the development of the first, you are likely to make comparisons. "Rover was house-trained in two weeks, but Fifi is still having trouble at 4 months! She just isn't getting it."

Another common reason for a second dog is to provide companionship for the first. While you are at work the two can play and investigate their environment together. The presence of another dog also can make the long hours of isolation more interesting. While creating a buddy system is a laudable goal, success is not guaranteed. Your expectations of the new dog are often greater than those imposed on the first. The second dog must build a strong relationship with the owner and the resident dog. This dual role is not as easy as it may seem. Some dogs have great people skills but lack dog skills. The worst reason to acquire a second dog is to fix any annoying behavior of your current dog. If Rover digs holes and barks constantly, another dog will not magically make him quit. The second dog may simply create additional holes and more noise. This combination of unruly pets and inattentive owner is usually fraught with problems. More likely, you'll raise the second dog by the same methods and expectations that shaped the unacceptable behavior of your first dog. If your goal is to have the second dog keep the first one company, you need to be honest about the first dog's personality. Does it like other dogs? Will it like the new dog you choose? Will it be satisfied sharing you with another dog? If your goal is to perk up an older dog, a puppy may not be the right choice. For a dog with declining health, a more sedate adult animal may make the perfect companion.

While puppies are unlikely to instigate aggression from an adult dog, it can happen. Choosing an adult companion allows you to know immediately if you have a friendly match. A general rule is to alternate the sexes of dogs. If you have a male, choose a female for your second dog and vice versa. While this may seem common knowledge, it is not always a predictor of success. Many male dogs live together in complete harmony. Some males will fight with females, and some females will fight males. The assumption that females are more passive doesn't always hold true. Some of the most vicious, determined aggression is between female dogs. If you have decided to add an infant animals, you will probably have an easy initial transition. Puppies and adults usually get along within a short time. Yet if a pup and older dog coexist easily, it does not ensure that you have made the perfect match. The fact that a puppy is friendly now does not mean it will remain that way--some undesirable behaviors are developmental. Male dogs tend to become aggressive at the advent of sexual maturity, about 9 months, or at social maturity, about 18 months. Be prepared to monitor the relationship as the younger animal reaches adulthood.

Adding a new dog to the family is a serious decision that should be well thought out beforehand. Knowing your own pet and understanding the role a new dog will play can help make the transition smooth and successful for everyone concerned.

(Gary Wilkes, author of the above article, is a respected animal behavior specialist, award-winning newspaper and magazine columnist, and the author of A Behavior Sampler.)