After a short introduction to the fundamental concepts of toxicology, the study of the effects of chemicals on biological systems will be approached, both through laboratory toxicological methods, either by different approaches by the use of biotic indices, bioindicators and biomarkers. Will be then describe the theoretical prediction models (QSAR and SAR), which are essential and increasingly being used for a preliminary assessment in all cases in which you do not have adequate experimental data. at last, will be outlined procedures for the setting, both for the individual substances and mixtures of toxic, environmental quality criteria. The second part of the course will be addressed to the environmental impact of potentially toxic substances. In the last part of the course will be described the procedures for environmental risk assessment and related risk indices through environmental monitoring. Finally, a mention will be made to the relationship between Ecotoxicology and social sciences (economics, politics, legislation) in decision-making of certain hazardous substances.