This PhD thesis consists of three parts to illustrate the goal of getting a better understanding of thefate and toxicity of saponins in the aquatic environment. It includes an introduction to the generalaspects of saponins, their chemistry and the ecotoxicology concepts, and a second part comprising thethree manuscripts based on the experimental work of the PhD focusing on the hydrolysis and toxicityof saponins. The third and final part consists of a conclusions and perspectives section of the wholePhD work.Saponins are a class of bioactive natural compounds. Due to their detergent-like structure, saponinshave a lot of applications, e.g. as foaming agents in consumer products, as adjuvants in the vaccine, asbiosurfactants in soil washing and as biopesticides in crop protection. Hence, they may leach into theaquatic environment due to their low octanol/water partition coefficient and poor binding to organicmatter. They may therefore also pose a risk to the aquatic organisms. Since saponins are efficientagainst pests, they are most likely also toxic to the non-target organisms. However, their fate andtoxicity in the environment are not fully understood. There are two main limitations of previous studies.One is that most studies are performed with the plant extract, which leads to the difficulty of comparingtoxicity data as the saponin exposure concentrations are not defined. Other compounds in the extract(other than saponins) may contribute to the toxicity as well. The other limitation is the analyticalmethod. It is currently difficult to measure and trace saponins at environmental concentration, due tothe large diversity of saponin structures that makes the separation difficult in the analysis. For example,there are more than 50 distinct saponins identified in the saponin fraction of Quillaja saponaria, with amolecular weight (MW) ranging from 1200 ~ 2300 g/mol. This thesis has worked with saponinsenvironmental fate and toxicity using three different approaches:Firstly, we aim to provide an example of how to monitor the natural compounds (saponin-richplant extract) in the environment, and to relate the measurements to the potential risk of the differentfractions (saponins and non-saponins) in the saponin-rich plant extract. The toxicity tests wereconducted with the aquatic crustacean Daphnia magna and zebrafish (Danio rerio) embryos. Weconfirm that the saponins cause the majority of the toxicity (85.1-93.6%) in the quillaja extract, by