Nanotechnology has made enormous progress over the last few decades, and the current use of nanomaterials is rapidly increasing. As a result, the continuous release of nanomaterials into the air, water, and soil has raised concerns about possible adverse consequences for environmental and human health. This Special Issue pays close attention to the evaluation of nanomaterials' ecotoxicity associated with their occurrence, behavior, fate, and bioavailability. Such an evaluation would be critical for scientists, legislators, business leaders, and the public to understand and develop effective solutions to the potential impacts of nanomaterials.