The aim of this chapter is to provide an overview on toxicity tests and bioassays and their current status of development and adaptation for the specific purpose of engineered nanomaterials (ENMs) toxicity testing involving organisms commonly used in environmental research. Specific challenges in toxicity testing of ENMs, such as properties, behaviour, transformations of ENMs in liquid media, consequences for attachment, bioaccumulation and kinetics in organisms and interferences with toxicity test components or with detection methods are addressed. The core of the chapter is a detailed presentation of toxicity tests including: unicellular organisms (microbes, algae, protozoa), invertebrates (crustaceans, mollusks, insects, nematodes), vertebrates (amphibians, fish), vertebrate cells (fish cells or cell line) and higher aquatic plants. For each of the test organisms, an overview of the scope, test parameters, common species used, advantages and limitations for ENMs testing, as well as challenges and needs for further optimization and developments are presented. Finally, the current state of the art regarding standardization of these tests for ENMs testing is presented.