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Monitoring of plastic pollution focuses on macroplastics (>2.5 cm), with microplastics (<5 mm) only quantified in ad-hoc research studies. When microplastics are studied, it is only in flagship species and little is known about food web interactions with microplastics. Using the River Thames as a case study, this thesis establishes an ecosystem-wide assessment of macro- and microplastics. Such an approach has yet to be utilised elsewhere. This thesis provides evidence for the trophic transfer of microplastics and, through examination of macroplastics, highlights potential sources of microplastics. Furthermore, the present research demonstrates that macroplastic monitoring alone is insufficient to identify all major sources of plastic in the environment. Macroplastic is dominated by single-use products, in particular food packaging with rainfall contributing to inputs of sewage-related debris. Microplastic concentrations in sediment range from 0.2 to 2.6 particles per gram, with fibres being most abundant both in biota and sediment. Microplastics are readily ingested, with marine mammals as top predators (e.g., Halichoerus grypus and Phocoena phoceona) consuming the most (102 items on average). When organism size is accounted for, however, Corophium volutator and Hediste diversicolor ingests higher concentrations. Therefore, benthic infauna are exposed to the greatest microplastic concentrations. Eriocheir sinensis (Chinese mitten crab) are also highly contaminated, accumulating large tangles of plastic in the gastric mill. Over 90% of individuals contain tangles, many of which were made of over 100 plastic fibres. Microplastic colour and polymer type varies between the environment and biota with a greater proportion of coloured fibres and cellulosic particles present in the digestive tract. The results suggest that microplastics can be transferred to predators, but biomagnification is unlikely. The evidence provided supports the use of an ecosystem approach to plastic monitoring in addition to the use of model species and can provide insight into products posing the greatest risk. This evidence can be used to influence policy, product design and disposal.