Dogs are considered "man's best friend" and on Friday they were also "man's best co-worker."

For the 20th year in a row, companies celebrated Take Your Dog to Work Day, an annual event started by the educational association for professional pet sitters and dog-walkers Pet Sitter's International. Other pets were not excluded — the entire week of June 18-22 was Take Your Pet To Work Week.

And while this pet-friendly week may have come to an end, more and more companies are allowing dogs in the workplace in an effort to attract millennials, such as Amazon and Google.

These policies are definitely a nice perk for animal-lovers, but there are larger health and wellness implications. Science shows that canine companionship can increase self-esteem and social support and reduces stress and anxiety levels.

These benefits are likely why many highly successful people in the fast-paced tech industry are quite fond of their furry friends.

From Mark Zuckerberg to Elon Musk, meet the dogs of eight tech titans: