Heat shock proteins (HSP) were first identified in cells after exposure to elevated temperature. Subsequently HSP have been identified as a critical component of a very complex and highly conserved cellular defence mechanism to preserve cell survival under adverse environmental conditions. HSP are preferentially expressed in response to an array of insults, including hyperthermia, free oxygen radicals, heavy metals, ethanol, amino acid analogues, inflammation and infection. HSP interact with intracellular polypeptides and prevent their denaturation or incorrect assembly. In addition HSP are also involved in several processes essential for cellular function under physiological conditions. HSP production is enhanced during in-vitro embryo culture and they are among the first proteins produced during mammalian embryo growth. The spontaneous expression of HSP as an essential part of embryo development is well documented and the presence or absence of HSP influences various aspects of reproduction in many species. Finally, HSP are immunodominant antigens of numerous microbial pathogens, e.g. Chlamydia trachomatis, which have been recognized as the main cause of tubal infertility. Many couples with fertility problems have had a previous genital tract infection, have become sensitized to microbial HSP, and a prolonged and asymptomatic infection may trigger immunity to microbial HSP epitopes that are also expressed in man. Antibodies to both bacterial and human HSP are present at high titres in sera and hydrosalpinx fluid of many patients undergoing in-vitro fertilization (IVF). In a mouse in-vitro embryo culture model, these antibodies impaired the mouse embryo development at unique developmental stages. Recent studies indicate an association between a previous infection, immunity to HSP and reproductive failure.