Butyrylcholinesterase (BuChE) is routinely measured to assess exposure to or effects of organophosphorus insecticides (OP). As a biomarker, it can be used to clarify the relation between exposure to OP and health impairment. The interpretation of BuChE inhibition data, particularly of small changes in enzymatic activity, sometimes presents significant complexities. These complexities are presented in this short communication and the factors that influence the degree of BuChE inhibition are discussed. Despite the complexities of their interpretation, BuChE measurements remain a mainstay for the fast initial screening of exposure to OP; thus, they are a useful tool in the protection of humans, domestic animals and wildlife from overexposure to these toxic agents.