Epiphytic diatoms perform a variety of ecological functions. Diatoms are important primary producers and sources of oxygen which can modify the chemistry of the surrounding aquatic environment. They may live attached to plant surfaces with the help of extracellular polymeric substances and compete with plants for resources (e.g., light, nutrients). Thus, they represent an excellent model system for studies on interactions between epiphytes and their host plants under different environmental conditions. Further, the practical usage of epiphytic diatoms in biomonitoring begs questions concerning substrate specificity, diatom biodiversity, and species delimitations. This review focuses on specific aspects of freshwater epiphytic diatom ecology as adaptations for epiphytic way of life, epiphyte-host relationships, and implications for biomonitoring.