The most discussed hazardous properties of chemicals with respect to environmental impacts include persistency, tendency to bioaccumulate in biota, human toxicity, and ecotoxicity to living biota. In this chapter, aimed primarily at students and non-professionals in ecotoxicology such as environmental engineers or remediation experts, brief introduction to ecotoxicology is provided. The chapter brings introductory insights into the concepts of ecotoxicology and effects of chemicals in biological systems including the discussion of concentration–response relationships. Further, we introduce methods and tools in the testing of ecotoxicity, i.e., bioassays, and discuss their applications in both prospective and retrospective hazard assessment. Finally, the chapter provides a simple overview of major contaminants and their documented ecotoxic impacts on living organisms in both aquatic and terrestrial ecosystems.