Molecular mimicry, the presence of shared epitopes between foreign and self antigens is common both at the B and T cell level. That it only rarely leads to a cross-reactive auto- immune response is a testimony to the effectiveness of the immune system in regulating the immune response and avoiding harmful self-reactivity. However, despite this regulation, various auto-immune-mediated diseases have been associated with molecular mimicry. The concept of molecular mimicry between foreign and self antigen as well as between self and modified self epitopes is reviewed and the relevance of these phenomena for the initiation of autoimmune diseases is discussed.