The formation and maintenance of boundaries between neighbouring groups of embryonic cells is vital for development because groups of cells with distinct functions must often be kept physically separated. Furthermore, because cells at the boundary often take on important signalling functions by acting as organizing centres, boundary shape and integrity can also control the outcome of many downstream patterning events. Recent experimental findings and theoretical descriptions have shed new light on classic questions about boundaries. In particular, in the past couple of years the role of forces acting in epithelial tissues to maintain boundaries has emerged as a new principle in understanding how early pattern is made into permanent anatomy. © 2011 Macmillan Publishers Limited. All rights reserved.