Antibodies detected by solid phase anticardiolipin immunoassays are heterogeneous but understanding the nature of this heterogeneity has proven difficult. Differences in any of the following features may account for differences in anticardiolipin antibodies: avidity for cardiolipin, phospholipid cross-reactivity, specificity for β2 glycoprotein I (β2GPI)-cardiolipin complexes, functional activity (as defined by the ability to inhibit phospholipid dependent coagulation reactions) and specificities for different conformations of phospholipids. Understanding these differences may be important in explaining the variations in clinical presentation of patients with these antibodies. This review attempts to define and discuss anticardiolipin heterogeneity.