Flame retardants (FRs) have come under considerable scientific and public scrutiny over the past decade. A lack of reference materials and standardized analytical methods has resulted in questions regarding the variation of measurements from different studies. We evaluated this variation by performing an international interlaboratory study assessing analytical capabilities as well as the accuracy and precision of results for a range of flame retardants (International Flame Retardant Laboratory Study, INTERFLAB). Thirteen international research laboratories participated in a blind interlaboratory comparison of 24 FRs. Results demonstrate good precision within replicates of test mixtures from individual laboratories, but problematic accuracy for several FRs and laboratories. Large ranges in the values reported for decabromodiphenylethane (DBDPE), tris(1,3-dichloropropyl)phosphate (TDCIPP), tetrabromobisphenol-A (TBBPA), and hexabromocyclododecane (HBCD) (>50 % relative standard deviations among measured values) and large deviations from the reference values (>25 % bias in accuracy) suggest potential problems for comparability of results. DBDPE, HBCD, and TBBPA had significantly poorer accuracy and precision, suggesting that current analytical methods are not providing reliable results for these FRs.