In 1990, HSP40 was identified as a novel mammalian heat shock protein with a molecular weight of approximately 40 kDa. Subsequent its partial amino acid sequence and cDNA cloning revealed that HSP40 is a homologue of bacterial DnaJ heat shock protein. It has been found that HSP40 together with HSP70 is involved in the protein folding, and in protection of heat denaturation of protein and promotion of the recovery of heat-denatured protein. It has also been found that HSP70 and HSP40 jointly inhibit the formation of protein aggregates characteristic of neurodegenerative diseases and also function in restoring the activity of temperature-sensitive mutant proteins. Currently, 49 HSP40 homologues are identified in mammals, and the term HSP40 is now used as a family name. This review describes the circumstances of the discovery of HSP40 and its subsequent progress. Although this review is a personal research history, it also provides a background of the fields of heat shock proteins and significant information for the development of hyperthermia research.