Immunoglobulin light chains are components of antibodies, but some exist in a free form in serum and urine as a result of their excess production over heavy chains. Free light chain (FLC) levels are of the order of milligram per liter in normal serum and urine, but marked increases have been observed in various disease conditions. It has now been established that the measurement of FLC levels contributes to diagnosis and clinical management in monoclonal gammopathies. Recent developments in FLC assays have been adapted to several automated platforms and they have now become available in laboratories. There have, however, been some concerns regarding the analytical aspects. The current assay specificity appears to be insufficient to prevent the influence of intact light chains of several orders of magnitude greater than FLCs in serum. Moreover, the heterogeneous nature of light chains makes accurate quantification unreliable. FLC assays have never been standardized because of the lack of an international reference calibrator. In this review, we summarize the reports on FLC measurements and examine the specificity of anti-FLC antibodies and the reliability of FLC assays. We also discuss difficulties in the standardization and setting of normal reference intervals for FLC assays.