Ubiquitination is a versatile tool of eukaryotic cells for controlling the stability, function and subcellular localization of proteins. The variety of cellular processes regulated by ubiquitination demands high substrate specificity of the ubiquitination machinery as well as the existence of diverse downstream effector proteins interacting with ubiquitinated substrates. Most of these cellular effectors are characterized by a modular composition of ubiquitin-binding motifs and further domains mediating specific functions. Here, I give an overview of important ubiquitin-related protein motifs, including the UBA, UIM, UBD and UBX domains, and propose a model for the role of subclasses of UBA-domain-containing proteins in ubiquitin-mediated proteolysis.