Nearly 163 breeds are represented in seven show categories. The grand prize is a trophy for "Best in Show" and bragging rights until next year.
Organizers say the top five dogs in the country are in town.
"We have a Pekinese here, a smooth fox terrier, an Irish setter, heavy competition vying for that number one spot," Marti Johnson from the San Mateo Kennel Club said.
The next big event on the canine calendar is the Westminster Dog Show coming up in New York in February.
If you want to see the dogs in San Mateo, admission is free and the show continues from 8 a.m.-5 p.m. Friday at the fairgrounds.