We are adopting a one-year old retriever mix from the shelter. What is the best way of introducing him to our other two-year old female dog?

Introduce them gradually.

Because both dogs are young, this transition will be a little less stressful for you than if the dogs were older. Introducing pets that are not too set in their ways usually makes things easier. It also helps that one dog is female; female dogs tend to be more accepting of outsiders than a more dominant male.

It is helpful to try to introduce them very gradually. It would be a good idea if the dogs could meet for the first time on neutral territory. Try taking them to a park or some location that is not on either one of their "turfs." Keep the dogs on leashes and allow them to sniff each other but do not give them the opportunity to fight. This interaction will give you a starting point. If there is aggression on neutral territory it may be quite difficult to move them in together. It will be very important for you to reinforce the dominance hierarchy once established. This means that when one dog establishes itself as the top dog and wants attention you must give it to the dominant dog and rebuff the other if it comes seeking attention. This may be hard for you to do as most people feel for the "underdog." Unfortunately, if you push the dominant dog away to pet the dog of lower status, it can cause the top dog to punish the other pet. In most cases there will not be an altercation, but be prepared for one anyway. Keep your pets on leashes even in the house. DO NOT GET BETWEEN FIGHTING DOGS. Even if your pets love you, when they are fighting they may bite you. Use their attached leashes to separate them. Remember that gradual introduction is the key. Do not expect them to be best friends right away; tolerance is a good thing too. If there are problems, the doctor is your best source of information. If the problems are significant he or she may also recommend that a short course of medication is necessary to facilitate the transition.