||All cells have distinct cellular architectures that are critical for their function. A dramatic example of this relationship can be observed in cells that undergo subcellular branching. In this case, cells must first specify distinct branch sites and then outgrow cellular projections from those sites, resulting in a branched cellular morphology. Branching morphology is a common type of cell shape, examples of which include glial oligodendrocytes found in the human brain, dendritic cells of the mammalian immune system and by far the best studied example, neurons. Tubulogenesis or lumen morphology is another type of cell formation common throughout biology. Cellular tubes transport liquids and gases within animal tissues and are often found in elaborate organ systems that span the entire body including the human respiratory and circulatory systems. Despite the importance of these forms of cell architecture, little is known about the genes and molecular machinery that are required for developing branched tubular cells. Drosophila larval tracheal terminal cells are single, highly branched cells that have a subcellular lumen running through each branch. These cells are located at the ends of a network of interconnected tubes and are the final, and critical step in delivery of oxygen and other gases to animal tissues. Terminal cell development, which occurs primarily during larval stages, includes three distinct morphological processes: cell growth, subcellular branching and tubulogenesis. Cell outgrowth is a general process that is used by many other cell types to enable the overall growth. Subcellular branching is a iv specialized process that includes sending cellular projections out from the plasma membrane towards other cellular targets. Lastly, tubulogenesis is the process of forming a space or lumen within a cell through which gas can flow. Here, we use Drosophila larval terminal cells, a component of the respiratory system, to investigate the cellular mechanisms required for development of two distinct cellular morphologies, subcellular branching morphogenesis and subcellular lumen formation. Work described here focuses primarily on progress made in elucidating mechanisms of branch specification and branch outgrowth. We have found that the PAR-polarity protein complex is required for terminal cell branching and that through the Rho GTPase Cdc42, and other PAR proteins, the exocyst facilitates polarized membrane addition required for terminal cell branch outgrowth.