For the person who accompanies another during social situations, see chaperon.

In biology, chaperones are proteins whose function is to assist other proteins in achieving proper folding. Many chaperones are heat shock proteins, that is, proteins expressed in response to elevated temperatures. The reason for this behaviour is that protein folding is severely affected by heat, and therefore chaperones act to counteract the potential damage. Although most proteins can fold in absence of chaperones, a minority strictly requires them.

A large number of chaperones need ATP for proper function. However, chaperones are quite diverse and there is still considerable debate and uncertainty on how they function. Chaperones recognize unfolded proteins by the hydrophobic residues these expose to the solvent. Exposed hydrophobic residues are unusual for properly folded proteins, which almost always hydrophilic amino acids only. Since the environment of the cell is characterized by hydrophilic groups (mostly water), incompletely folded proteins or misfolded proteins with exposed hydrophobic groups have a tendency to aggregate. This aggregation is extremely detrimental to the cell, and chaperones help to prevent this by providing encapsulated hydrophobic environments that allow the protein to properly fold.

Chaperonins are a subset of chaperone proteins found in prokaryotes, mitochondria and plastids.