Major histocompatibility complex class II molecules (MHC II) have developed a particular trafficking pathway, which allows them to load antigenic peptides generated in the endosomal/lysosomal system. Following the original description of MHC II associated invariant chain (Ii) in 1983 [1, 2], evidence has accumulated demonstrating that its binding to MHC II K and L chains is crucial at several steps of MHC II biogenesis and function [3]. Despite the small size of Ii, this protein comprises an impressive and still growing number of functions (see Table 1). Here, we will review the different roles of Ii, while following the journey of nascent MHC II complexes through the cell. Special attention will be given to the molecular events taking place in the endoplasmic reticulum (ER), during the transport to and within the endocytic pathway.