Although plastic has unquestionable benefits to modern society, its current use leads to pollution of the environment. Via direct disposal and degradation of larger plastic items, microplastic and nanoplastic particles reach the environment. Potential negative effects include ecological harm to species due to particle effects of plastic and effects of plastic on the transfer of chemicals between organisms and the environment. The particle characteristics as well as many environmentally relevant processes affect the fate of micro- and nanoplastic particles, the exposure, effects and consecutive risk to organisms. This thesis addresses a great number of these characteristics and processes in depth from the riverine towards the marine environment and combines these with other fate and effect studies from the literature to derive a provisional quantitative analysis of the ecological risk of micro- and nanoplastic in the aquatic environment.