An important function of the endoplasmic reticulum is the folding of newly synthesised polypeptide chains. Several general and specialised chaperones operate in the endoplasmic reticulum to assist protein folding. This book reviews our current knowledge of molecular chaperones of the ER, including the HSP70 class chaperones BiP and GRP170, their DnaJ and GrpE co-chaperones, the HSP90 class chaperone GRP94, and the lectin chaperones calnexin, calmegin, and calreticulin. This book provides an overview of the biochemistry and physical chemistry of molecular chaperones with emphasis on molecular chaperones of the endoplasmic reticulum.