The regulated trafficking of receptors and their associated ligands through the mammalian endosomal system is a process fundamental to the maintenance of cellular homeostasis. Whilst a great deal is known about the mechanisms by which receptors are internalised at the plasma membrane, the molecular machinery involved in their sorting, segregation and trafficking in the endosomal system is largely undescribed. Recently, a pentameric protein complex, called the retromer, was revealed to play a key role in the endosomal trafficking of a number of cargo molecules both from the cell surface and the trans-Golgi network (TGN). This thesis expands upon our understanding of the mammalian retromer complex by defining novel subunits and examining their role in endosomal trafficking. To begin, this study describes the identification and characterisation of a previously undescribed mammalian retromer subunit, mVps26B. Whilst able to interact directly with the core of the mammalian retromer complex, mVps35, we find that, unlike its paralogue, mVps26A, mVps26B is not recruited to early endosomes. Rather, mVps26B and other retromer subunits are recruited to actin-rich extensions of the cell surface (lamellipodia) expanding the role of the retromer from endosomal trafficking to include cellular motility. In addition to being the motile unit of the cell surface, lamellipodia or membrane ruffles are the site at which macropinocytosis occurs. Macropinocytosis is the engulfment of bulk quantities of extracellular fluid and nutrients via the dynamic invagination of membrane ruffles. This study provides the first insight of the role of the retromer in macropinocytosis. Firstly, we reveal that SNX5, another novel retromer subunit, is recruited to membrane ruffles following EGF treatment. This may reflect the unique specificity of the SNX5 PX domain for the plasma membrane-derived PI(3,4)P2. Secondly, we reveal that SNX5 is recruited to discrete patches on the cytosolic face of the nascent macropinosome almost immediately following its ...