A sister city, county, or state relationship is a broad-based, long-term partnership between two communities in two countries. A relationship is officially recognized after the highest elected or appointed official from both communities signs off on an agreement to become sister cities.

A city may have any number of sister cities with community involvement ranging from just a few to hundreds of volunteers. In addition to volunteers, sister city organizations often include representatives from nonprofits, municipal governments, the private sector and other civic organizations.

Each sister city organization is independent and pursues the activities and thematic areas that are important to them and their community including municipal, business, trade, educational and cultural exchanges and projects with their sister city.

Sister Cities International is a nonpartisan nonprofit organization that serves as the national membership organization for individual sister cities, counties and states across the United States. This network unites tens of thousands of citizen diplomats and volunteers in nearly 500 member communities with over 2,000 partnerships in more than 140 countries.