According to various sources, the dog breed with the longest lifespan is the Chihuahua, with an average lifespan of 14-20 years. However, it is important to note that not all dogs of the same breed will live the same length of time, and averages can only provide guidelines for what to expect. Other dog breeds that are known for their longevity include the Maltese, Bichon Frise, Yorkshire Terrier, Beagle, Pomeranian, and Dachshund. It is generally believed that smaller dogs tend to live longer than larger breeds.