In 1995, in the Biochemical Society Transactions, Mundy published the first review on CLIMP-63 (cytoskeletonlinking membrane protein 63) or CKPA4 (cytoskeleton-associated protein 4), initially just p63 [1]. Here we review the following 20 years of research on this stillmysterious protein. CLIMP-63 is a type II transmembrane protein, the cytosolic domain of which has the capacity to bind microtubules whereas the luminal domain can form homo-oligomeric complexes, not only with neighbouring molecules but also, in trans, with CLIMP-63 molecules on the other side of the endoplasmic reticulum (ER) lumen, thus promoting the formation of ER sheets. CLIMP-63 however also appears to have a life at the cell surface where it acts as a ligand-activated receptor. The still rudimentary information of how CLIMP-63 fulfills these different roles, what these are exactly and how post-translational modifications control them, will be discussed.