Microplastics have emerged as widespread contaminants in freshwater ecosystems. However, their sources, environmental behaviour, and ecotoxicological impacts remain under-characterised relative to marine environments. This chapter provides a comprehensive review of microplastic pollution in freshwater systems, encompassing rivers, lakes, lagoons, and reservoirs. Key sources of microplastics were identified, including urban run-off, wastewater discharges, agricultural activities, and in situ fragmentation of larger plastic debris. Atmospheric deposition and stormwater drainage also play significant roles in the mobilisation and transport of microplastics into inland waters. Reported concentrations vary widely across freshwater systems, influenced by local land use, hydrodynamics, and methodological discrepancies in sampling protocols. The composition of microplastics is predominantly characterised by fibres derived from synthetic textiles and common consumer polymers such as polyethylene, polypropylene, and polyethylene terephthalate. Microplastics are found throughout the water column, sediments, and aquatic biota, with their spatial distribution and fate determined by factors such as polymer density, biofouling, and sediment-water interactions. Ecotoxicological evidence indicates that microplastic exposure can lead to diverse sublethal and chronic effects in freshwater organisms, including behavioural alterations, oxidative stress, impaired reproduction, growth inhibition, and endocrine disruption. Furthermore, microplastics act as vectors for co-contaminants, including persistent organic pollutants and heavy metals, thereby enhancing their ecological risk. Trophic transfer has been documented across multiple freshwater species, suggesting the potential for biomagnification within aquatic food webs. However, most studies have focused on simplified laboratory models, leaving critical knowledge gaps regarding long-term ecological impacts and implications for larger, commercially important species. The findings underscore the urgent need for refined ecotoxicological frameworks that prioritise chronic exposure scenarios and consider the complex physical and chemical characteristics of microplastic pollution. Future research should aim to integrate standardised methodologies, evaluate trophic transfer dynamics, and assess the combined effects of microplastics and co-occurring contaminants to inform risk assessments and policy interventions.