A correct three-dimensional structure is a prerequisite for protein functionality, and therefore for life. Thus, it is not surprising that our cells are packed with proteins that assist protein folding, the process in which the native three-dimensional structure is formed. In general, plasma membrane and secreted proteins, as well as those residing in compartments along the endocytic and exocytic pathways, fold and oligomerize in the endoplasmic reticulum. The proteins residing in the endoplasmic reticulum are specialized in the folding of this subset of proteins, which renders this compartment a protein-folding factory. This review focuses on protein folding in the endoplasmic reticulum, and discusses the challenge of oligomer formation in the endoplasmic reticulum as well as the cytosol.