The anticardiolipin (aCL) test has been widely used by physicians since the mid-1980s for diagnosing patients with antiphospholipid syndrome (APS). Establishment of this diagnosis has enabled effective management of patients with recurrent thrombosis and recurrent pregnancy losses. The test was first established in 1983 as a radioimmunoassay and soon thereafter converted into an enzyme-linked immunosorbent assay (ELISA). The other test commonly used in the diagnosis of APS is the lupus anticoagulant (LA) test. The aCL ELISA is sensitive for the diagnosis of APS but lacks specificity. On the other hand, the LA assay, although more specific, is not as sensitive as the aCL ELISA. More specific tests are now available such as the anti-beta2 glycoprotein I (anti-beta2GPI) assay, the antiprothrombin assay, and other ELISAs that use negatively charged phospholipids instead of cardiolipin to coat the plates. In the past 25 years, there have been numerous efforts to standardize aCL, LA, and anti-beta2GPI tests but there are still reports of significant intra- and interlaboratory variation in results for all three assays. This article discusses in detail the clinical value of these tests, technical problems associated with their use, the current laboratory classification criteria for diagnosis of APS, and possible new and better assays that will be available in the near future for diagnosis of APS.