Meet the adventurous couple who has spent the past 4 years living in Airbnbs around the world for under $90 a night

Since July 2013, retirees Michael and Debbie Campbell have lived in 150 different homes around the world, all booked through short-term rental service Airbnb.

On July 22, they spent their 1,000th night in an Airbnb in Strasbourg, France, the same country where they began their journey four years ago.

Throughout their Airbnb-hopping adventure, the Campbells have become a poster couple for the Silicon Valley-based travel company. They've visited Airbnb offices in Portland, San Francisco, and Dublin, spoken at a company conference in Paris, and penned a manual for Airbnb hosts called "Your Keys, Our Home." They've also mastered the skill of traveling on a budget, sticking to a $90 nightly rate and finding the best free and low-cost activities in each city they visit.

Business Insider recently caught up with Michael, 71, and Debbie, 61, to learn more about their ongoing trip around the world.

Scroll through to learn more about how they do it, and follow their adventure on their website, Senior Nomads.