The no-observed-adverse-effect-level (NOAEL) is an important part of the non-clinical risk assessment. It is a professional opinion based on the design of the study, indication of the drug, expected pharmacology, and spectrum of off-target effects. There is no consistent standard definition of NOAEL. This is based, in part, on the varied definitions of what constitutes an adverse effect. Toxicologists, either investigating or reviewing, have not been consistent in defining an effect as either adverse or acceptable. The common definition of NOAEL, "the highest experimental point that is without adverse effect," serves us well in general discussions. It does not, however, address the interpretation of risk based on toxicologically relevant effects, nor does it consider the progression of effect with respect to duration and/or dose. This paper will discuss the issues and application of a functional definition of the NOAEL in toxicology evaluations.