A versatile collection of readily reproducible cell-cell interaction assays for uncovering cellular interactions at the molecular level, both in vitro and in vivo. The protocols cover a diverse set of cell-cell interaction models in both normal and pathological states, are readily adaptable to nearly any cell type and organ system, and include primary data and outcome analysis. In addition, the protocols follow the successful Methods in Molecular Biology (TM) series format, each offering step-by-step laboratory instructions, an introduction outlining the principles behind the technique, lists of the necessary equipment and reagents, and tips on troubleshooting and avoiding known pitfalls.