While plastic pollution in marine environments is getting better known and is the subject of a continuously growing number of publications, fresh- waters still receive too little attention. Some studies worldwide focused on the nature and concentrations of microplastics in freshwater bodies, but data is so far very limited and fragmentary. Microplastics were studied in Swiss surface waters since 2012, first in order to show the reality of this contamination, and later to better understand its nature and extent. Microplastics were found in significant concentrations in all the sampled lakes and rivers, both on the water surface, beach sediments, and benthic sediments, as well as in organisms (fishes, birds, and zebra mussels). They were shown to contain potentially toxic additives, as well as to adsorb hydrophobic contaminants (Faure et al., 2015). Some pathways into the water bodies have then been studied including Waste Water Treatment Plant (WWTPs) urban runoff waters, soils or storm overflows. Potential sources and pathways of (micro-) plastics in the Lake Geneva watershed are now the focus, on the basis of the fractions of plastics that enter the environment at all stages of their life cycle depending on their use, and on the plastic types and quantities that can be found in the different environ- mental compartments. This dual approach of material flow modeling and a strong integration of measured environmental data is the first step towards producing a mass balance of microplastics in the Lake Geneva watershed.