The increasing uses of commercial chemicals have forced governments and international organisations, such as OECD, EEC etc. in the years following World War II to address the problem of toxic chemicals in the environment. The scientific basis for any chemical regulation is the knowledge of potential fate and effects of chemicals. Practically every chemical law which has been developed in recent years requests manufacturers and/or importers to register (notify) their production of their marketing of new chemicals to national authorities. The notifications comprise series of data on physico-chemical properties and biological (i.e. toxicological and ecotoxicological) characteristics of the chemicals. From such data it is attempted to establish a hazard ranking (or preliminary hazard assessment) of chemicals. This involves evaluation, or possibly calculation, of the expected exposure from the chemicals and of the potential effects resulting from such exposure. Some basic knowledge on the target, i.e. the ecosystem need to be included in the evaluation process. The possible use of mathematical modelling in hazard ranking and hazard assessment is presently confined to the exposure analyses, which are based on calculations from physico-chemical data of the chemicals and from parameters describing over-all properties of a so-called Unit World. In contrast, the potential effects of a chemical is assessed from a limited number of toxicological and ecotoxicological laboratory tests. The main criteriae used in hazard ranking are established from laboratory data on degradability, mobility and potential for bioaccumulation of the individual chemicals. Such data are selected as the fate (exposure) characteristics of primary importance, while information on acute toxicity to rat, fish, and daphnia are the measures used for evaluation of potential effects. In various ranking schemes these characteristics are expressed in simple numerical scales, and classification codes are assigned to each. These can be scoring systems, or - even more simple - descriptive codes as for instance colour codes, such as Black for ‘Damaging to the environment’, Grey for ‘Dangerous to the environment’, and White for ‘Potentially dangerous to the environment’. The limitations of presently known hazard rankings schemes are discussed, and it is suggested that basic chemical properties (such as the OECD/EEC base-set data or similar) should not realistically be utilised beyond the stage of simple classification rules, e.g. for labelling purposes. More elaborate hazard assessment schemes are still to be expected, and they should be based on more individually developed case-by-case studies.