The global pollution of aquatic environments with microplastics and their interaction with wildlife is of concern. Ingestion of plastic has been reported for a wide range of species but little is known about the potential effects of such ingestion. The aim of this thesis was to assess the biological effects that are induced by the ingestion of microplastic in the three-spined stickleback (Gasterosteus aculeatus). Novel data for the ingestion, retention, egestion and induced effects of microplastic in multiple ontogenetic life stages are presented. Microplastics of different types and sizes were found to be ingested from the water and diet. Additionally, trophic transfer of microplastic was found as a further route for ingestion. Whereas ingestion of relatively small plastic was not found to induce blockages or obstructions of the gastrointestinal tract, ingestion of relatively large plastic had the potential to result in prolonging effects on food digestion. However, all used microplastics were found to be egested successfully. Effects on length, weight and condition index K were found but varied between exposure types and data suggests that observed short term effects were induced by plastic associated chemicals. Whereas molecular analysis of cytochrome P450 1A confirmed exposure to xenobiotics, relative expression of vitellogenin indicated no exposure to oestrogenic, plastic derived chemicals. Degraded polymer structures showed to have a higher biological activity due to enhanced leaching of endocrine disrupting, plastic derived chemicals and showed a potential to disturb energy metabolism. In addition, plastic was found to act as vectors for absorbed bisphenol A from the water column via trophic transfer to stickleback larvae where desorption of accumulated chemicals had the potential to induce toxic effects. The data presented in this thesis indicate that microplastics can be ingested and can induce negative effects in multiple ontogenetic life stages of sticklebacks.