This introductory chapter highlights the applications of quantitative structure-activity relationships (QSARs) in the assessment of ecotoxicological risk posed by the chemicals used in our day-to-day life and in the industries. A wide variety of chemicals (industrial substances/toxicants/pollutants) are emitted into the environment from various sources. These chemicals may be pharmaceuticals, personal care products, nanomaterials, plasticizers, flame retardants, endocrine disruptors, pesticides, persistent organic pollutants (POPs), etc. The continuous emissions of chemicals into the environment and the resultant pollution effects and potential exposure of living organisms and humans to these noxious substances may pose a risk to the ecosystem and human health. The experimental determination of toxicities of these chemicals involving different aquatic organisms and laboratory animals is a lengthy, time-consuming, and costly process. In this scenario, QSAR is quite useful for the prediction of toxicities of these chemicals prior to their use on a large scale. QSAR models could also be used further to predict the toxicity of any designed chemicals and would thus be helpful for green chemical design.