Considering everything between macroplastics and nanoplastics, this chapter aims to provide a comprehensive and detailed assessment of the available methods to monitor and model plastics in freshwater bodies and to visualize the geographical distribution of studies reporting plastics in rivers and lakes. Both in marine or freshwater ecosystems, five steps can be distinguished for monitoring, namely, sampling, extraction, analysis, identification, and extrapolation. Despite the rapidly increasing number of publications on plastic occurrences in freshwater bodies, several aspects still carry high levels of uncertainty. The chapter highlights four key aspects that deserve more attention when designing and performing monitoring campaigns and when developing and interpreting numerical models. These four aspects are: temporal variation, transfer between environmental compartments, harmonization of monitoring and modeling investigations regarding plastic size ranges, and, finally, data quality control and validation of results.