The COMMD proteins are a conserved family of proteins with central roles in intracellular membrane trafficking and transcription. A hallmark feature of the COMMD protein family is a highly conserved C-terminal region, with no known structure, known as the COMM domain. The N-terminal domain of these proteins is highly variable and is proposed to ascribe unique functions to each of the 10 family members. In addition, it is known, that COMMDs form oligomeric complexes with each other and act as components of a larger assembly called the CCC complex, which is localized to endosomal compartments and mediates the transport of several transmembrane cargos. How these complexes are formed however is completely unknown. Here, we have systematically characterised the interactions between several human COMMD proteins and determined structures of COMMD proteins using X-ray crystallography and X-ray scattering to provide insights into the underlying mechanisms of homo- and heteromeric assembly. Our structural analysis revealed that, COMMD proteins possess an a-helical N-terminal domain. While the highly conserved C‑terminal COMM domain is composed of two cone shaped chains that are tightly intertwined with each other to form a globular dimeric module. Additional, biochemical analysis demonstrated that this dimeric COMM domains binds directly to CCDC22 and 93, key components of the CCC complex, via a conserved site. Finally, we were able to demonstrate that members of the COMMD family are able to bind nonspecifically to a wide variety of phosphoinositides. Overall, these studies show that COMMD proteins function as obligatory dimers with conserved domain architectures.