An important question in developmental biology is how a developing tissue grows and gets patterned. A key role in this process was attributed to secreted diffusing molecules, morphogens. Decapentaplegic (Dpp), a member of the TGF-β superfamily of ligands, has been shown to act as a morphogen and to control growth of the Drosophila wing imaginal disc. Dpp is secreted from source cells and spreads in the target tissue forming a graded spatial concentration profile. The growth of the Drosophila wing disc correlates with the growth of the Dpp gradient: both its amplitude and its decay length increase. The fine adjustment of the morphogen gradient to the size of the growing tissue is termed "scaling". The goal of the present thesis is to understand the Dpp gradient scaling at the cellular and molecular level. The shape of the Dpp gradient is determined by the dynamics of Dpp transport rates at the tissue level. In order to understand the mechanism of scaling, it is therefore important to understand what links the growing size of the tissue and Dpp transport.