We are interested in the mechanisms used by cells to arrange themselves into complex three-dimensional tissues. The precise spatial assembly of cells depends on the ability of cells to recognize, choose, and differentially adhere to neighboring cells. We are studying these processes in the wing imaginal disc of Drosophila. Imaginal discs are excellent systems to address developmental and cell biological questions. They are single cell-layered epithelial sheets amenable to molecular and cell biological approaches developing in the genetically tractable fly. Cells of the wing imaginal disc are arranged according to their lineage into two adjacent, but non-intermingling sets of cells termed compartments. Boundaries between compartments play vital roles for the partitioning and patterning not only of imaginal discs but also for vertebrate tissues. We use genetics and molecular approaches to identify molecules required for the formation of boundaries and to investigate how these molecules control and implement the sorting out of cells from adjacent compartments. In addition, we use imaging to observe the segregation of cells in real-time to gain insights into the underlying mechanisms.