This Guidance Document addresses aspects of OECD Test Guideline 305 (TG 305) on Fish Bioaccumulation. Following major revisions to TG 305 in 2012, it became clear that separate and detailed guidance was necessary to address complex areas of fish bioaccumulation testing, data treatment and interpretation. The aim of this document is to give guidance to the experimenter and user of the bioconcentration or bioaccumulation data on how to perform the test, calculate the results and interpret them. This guidance document should be seen as an explanation to the revised OECD TG 305, not as a substitute for it.