Measles virus possesses a non segmented, single stranded, negative sense RNA genome that is encapsidated by the nucleoprotein to form a helical nucleocapsid. This ribonucleoproteic complex is the substrate for both transcription and replication. The RNA–dependent RNA polymerase binds to the nucleocapsid template via its co-factor, the phosphoprotein. This book focuses on the main structural information available on the nucleoprotein, showing that it consists of a structured core (NCORE) and of an intrinsically disordered C-terminal domain (NTAIL). The functional implications of the disordered nature of NTAIL are discussed in light of the ability of disordered regions to establish interactions with multiple partners, thus leading to multiple biological effects. Indeed, beyond the phosphoprotein, NTAIL also interacts with cellular partners, including the major heat shock protein, hsp72, the interferon regulator factor 3, IRF3, and a yet unidentified cellular receptor referred to as NR. This book consists of two chapters devoted to the general functions of the nucleoprotein in transcription and replication and to a detailed overview of its structural properties, and of three chapters focused on the functional relevance of the interaction between NTAIL and its various intracellular and extracellular partners.