The wide use of pesticides in recent years has caused concern about the fate of the chemicals in the environment. Usage of pesticides in agriculture has potential hazardous impact on aquatic organisms since they are often concentrated in lakes and ponds through the agricultural runoff and later affect the aquatic sources adversely. Plankton which comprises a large portion of living matter in natural waters is affected to a greater extent. Toxicity is a biological response and this needs to be taken into account in formulating realistic guidelines on the acceptable upper limits on pesticide contamination of the environment. Toxicity testing has a clear role in safeguarding environmental quality but a considered selection of the testing methods is essential for obtaining relevant results. Keeping this in view, the present book focuses on the limno-biology and methods of ecotoxicological studies on aquatic biota and response of the aquatic biota to the toxicants. This book will be helpful for the students, researchers and environmentalists to get into limno-biology and ecotoxicological studies.