This book reviews knowledge on the interconnection of signal transduction and endocytosis/intracellular trafficking. The chapters cover knowledge obtained by using different model systems. The first chapter deals with Receptor Tyrosin Kinases (RTKs) with emphasis on the Epidermal Growth Factor Receptor (EGF receptor) and the Platelet Derived Growth Factor Receptor (PDGF receptor). The second chapter deals with the RTK c-Met and with how this RTK becomes carcinogenic. The third chapter reviews recent understanding on the mechanisms of action of the numerous fibroblast growth factors and their receptors. In the fourth chapter we learn about the trafficking of and signalling from the Growth Hormone Receptor and how this receptor is controlled by ubiquitination. The fifth chapter is devoted to the Interleukin II receptor, essential for activation of T cells. Links between ubiquitination, signalling, endocytosis, and sorting are reviewed. The last chapter discusses current views on how monoubiquitination controls both signalling and trafficking and thereby the final outcome of receptor activation.