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Assessment of the toxicological risk from foreign substances in the environment is based on the determination of action thresholds below which effects injurious to health no longer occur, even with life-long intake. According to the latest theories it is likely that such thresholds exist even for “irreversible” (carcinogenic, mutagenic) effects. However, the difficulties besetting the determination of threshold values by experimental or epidemiological methods still appear to be virtually insuperable. The problems are both quantitative (excessive numbers of animals and observations required) and qualitative (poor predictability of effects owing to substantial differences in biological reactivity between species). Neither animal experiments nor experience with human subjects can guarantee total security from toxic effects. Legislative measures designed to restrict the introduction of chemical substances into the environment must therefore be based on compromise. Scientists must be consulted, but ultimately the decisions must be political, and politicians have to assume the responsibility for them.