The chapter discusses the importance of aquatic invertebrates in river ecosystems, highlighting their diverse adaptations to varying environmental conditions such as water flow, oxygen levels, and food availability. These organisms play crucial roles in energy transfer within food chains, acting as primary consumers that feed on algae, microorganisms, and organic detritus, which in turn serve as food for predators. They also function as bioindicators, reflecting the ecological integrity of river systems. In Portugal, aquatic invertebrates have been extensively studied, with research focusing on their regional distribution, ecology, and use as bioindicators. The chapter outlines the diversity of aquatic invertebrates, including macroinvertebrates such as planarians, nematodes, annelids, mollusks, and arthropods like insects. It also discusses the functional groups of invertebrates based on their feeding habits and habitats, emphasizing their roles in nutrient cycling and energy flow. The text highlights the importance of invertebrates in maintaining ecosystem health and the threats they face, including habitat degradation, pollution, and invasive species. Endemic species are also discussed, noting their unique adaptations and the threats they face due to habitat loss and human activities. The chapter concludes with the significance of invertebrates as bioindicators and the need for conservation efforts to protect these vital components of river ecosystems.The chapter discusses the importance of aquatic invertebrates in river ecosystems, highlighting their diverse adaptations to varying environmental conditions such as water flow, oxygen levels, and food availability. These organisms play crucial roles in energy transfer within food chains, acting as primary consumers that feed on algae, microorganisms, and organic detritus, which in turn serve as food for predators. They also function as bioindicators, reflecting the ecological integrity of river systems. In Portugal, aquatic invertebrates have been extensively studied, with research focusing on their regional distribution, ecology, and use as bioindicators. The chapter outlines the diversity of aquatic invertebrates, including macroinvertebrates such as planarians, nematodes, annelids, mollusks, and arthropods like insects. It also discusses the functional groups of invertebrates based on their feeding habits and habitats, emphasizing their roles in nutrient cycling and energy flow. The text highlights the importance of invertebrates in maintaining ecosystem health and the threats they face, including habitat degradation, pollution, and invasive species. Endemic species are also discussed, noting their unique adaptations and the threats they face due to habitat loss and human activities. The chapter concludes with the significance of invertebrates as bioindicators and the need for conservation efforts to protect these vital components of river ecosystems.