Increasingly, ethnic and religious variables are taken into account to explain conflict and relations between nations. However, ethnic and religious groups exist beyond the confines of frontiers. In Africa, for example, hundreds of ethnic groups were divided by colonial borders, and many retained kinship connections to their brethren in other countries, thus creating “cross-border ethnic/religious affinity.” Such cross-border connections affect a variety of foreign policy, from diplomacy to the use of force. An internal problem can spread to other states, or external actors can become involved in domestic disputes due to such factors. Therefore data on cross-border connections are essential to measure and assess their actual or potential effects on foreign policy or conflict. This unique resource provides a detailed account of cross-border ethnic and religious affinities that will serve both qualitative and quantitative researchers. For ease of use, it is divided in sections for each region of world, with the entries organized by pairs of contiguous countries. Each entry for a pair of countries briefly discusses the various ethnic and religious groups living in each country, what groups are common to both countries, the historical connections between groups, how they interact with each other, and their impact on diplomacy and conflict. These entries are organized based on the Correlates of War (COW) country codes. These are widely used by researchers and allow for dyads to be organized geographically within each section to facilitate finding data.