The presence of antiphospholipid antibodies in plasma is a risk factor for thrombo-embolic complications. In vitro, however, the same antibodies can prolong clotting times in coagulation assays, a classic marker for a bleeding tendency. For years this contradiction puzzles many scientists.We now know that the term antiphospholipid antibodies comprises a heterogeneous population of antibodies and there is growing evidence that only subpopulations of antiphospholipid antibodies are relevant for the clinical complication. In combination with new information on the complex interaction between antiphospholipid antibodies, the protein beta2-Glycoprotein I, and cellular surfaces have opened new avenues for the understanding of the pathology of this syndrome.