Nanomaterials (NMs) already showed their great potential for improving the detection of trace contaminants and even the extraction of pollutants from the environment. However, NMs may also pose an environmental risk when released from their original matrices. When entering any biological environment NMs are covered by biotic and abiotic (bio)molecules ((eco)coronas), which is influenced by the NMs physicochemical properties. This in situ transformation of the NMs includes binding of natural colloids, which can serve as vectors for the NMs transport in natural media. Hence, the environment is not facing the pristine but rather the natural-coated engineered NMs ((eco)coronas). The field begins to accept that the biotic and abiotic coatings impacts not only the fate and transport of the released NMs but also their bioavailability and toxicity in the ecosystem. Unfortunately, current research programs and standardization protocols have mostly neglected this aspect so far. This chapter focus on the environmental fate of NMs, which determines how the NMs are biogeochemically processed and which receptors are most exposed (organisms and exposure route), with a special focus on their dynamic speciation and bioavailability, including a critical evaluation of the challenges and available analytical techniques to determinate NMs in complex environmental samples as well as computer codes and models.