The phrase "When in Rome, do as the Romans do" is an idiomatic expression that means you should act or behave in the way that the people around you do, especially when you're in a foreign place or culture. It suggests adapting to local customs and traditions when visiting a different country or region. The concept of adopting local customs is often referred to as "when in Rome" or "when in Rome, do as the Romans do."

This saying is attributed to St. Ambrose, who was explaining to St. Augustine why he should fast on Saturday, which was a Roman custom. However, it's become a widely recognized proverb used in many cultures to advise travelers and visitors to respect and follow local customs and practices.