Oral tolerance (OT) consists of the oral administration of antigens (Ag) that could alter the response of the immune system. This is a form of peripheral immune tolerance in which mature lymphocytes in the peripheral lymphoid tissues are rendered non fonctional or hyporesponsive by prior oral administration of Ag. It was first described in 1911 in animal models of anaphylaxis. This therapeutic approach requires the orally administration of Ag and the active participation of the gut-associated lymphoid tissue (GALT), a tissue comprising Peyers patches, intraepitelial cells and villi containing epithelials cells which is a well organized immune network. The mechanisms by which OT is mediated included deletion or anergy and active cellular suppression. The primary factor determining which form of tolerance will be developed after oral administration of Ag is the Ag dosage. Thus, it is thought that low doses of Ag induce the generation of active suppression, via regulatory T cells in the GALT, which then migrate to the systemic immune system. These regulatory T cells produce down-regulatory cytokines such as IL4, IL10 and TGFβ, a Th2 / Th3 cytokine pattern. Conversely, high dose of Ag favors anergy or clonal deletion. The phenomenon in which regulatory cells, as generated by oral tolerization, are primed in an Ag specific manner, but act in the respective microenvironment in a non-Ag specific manner is called bystander suppression. This phenomenon is of particular interest and explained the use of OT in T cell mediated autoimmune diseases such as rheumatoid arthritis (RA), multiple sclerosis (MS) and type I diabetes, some diseases in which the aut oAg remains unknown or where there are reactivities to multiple autoAgs. There were several studies demonstrating the effectiveness of orally administered Ag in different animal models of autoimmune diseases, such as experimental allergic encephalomyelitis, collagen induced arthritis, diabetes, but also uveitis, myastenia gravis and transplantation. These mouse or rat models of autoimmune diseases gave the rationale for the therapeutic use of OT in human and this therapeutic approach has been tried in MS and RA, using oral myelin or oral collagen, respectively. In RA, 4 trials of oral type II collagen (CII) in RA have been published. Taken together, these studies suggested that oral CII in RA gave a trend toward clinical improvement, with significance in only 2 studies. Bacterial extract from Escherichia coli containing heat shock proteins has been tried in oral treatment for RA. Two placebo controlled trials and 2 comparative studies gave favorable results for this bacterial extract with no or mild adverse events. Altough oral / mucosal tolerance has given successful results in animal models of autoimmune diseases, the enthusiasm for this therapeutic approach in human diseases must be tempered. The discrepancies between the effectiveness of OT in animal models and the resullts in human trials raise some questions, the identification of the subgroup of patients who might respond to this treatment and the source (or nature) of the administered Ag (homologous versus heterologous), for instance.