Claudins form the paracellular tight junction seal in epithelial tissues. Although there is still limited information on how these proteins are organized at the junction, a number of recent studies have provided useful insights both into claudin-claudin interactions and into interactions between claudins and other proteins. The focus of this review is to summarize recent information about claudin interactions and to identify critical unanswered questions about claudin organization and tight junction structure which will be required to understand claudin function.