Most of the knowledge concerning the intracellular mechanisms involved in cell locomotion have been obtained from in vitro studies of cells in culture. Many of the concepts derived from these studies have been partially confirmed in in vivo systems but numerous questions regarding the developmental control of cell migration remain to be addressed. Tracheal morphogenesis in Drosophila melanogaster embryos represents an in vivo model system to study the genetic control of cell migration. We review what is known about tracheal development and regulation of tracheal cell migration. We try to link these in vivo studies and the movement of cells over two dimensional substrates and elaborate on important questions which remain to be addressed in the future.