Ferns belong to the oldest evolutionary lineages of vascular plants, yet they remain a relatively poorly understood group in terms of phytochemistry and biological activity. In recent years, the flavonoids found in ferns have attracted increasing interest due to their important physiological role in plants and their potential health benefits. The aim of this article is to present the current state of knowledge regarding flavonoids present in ferns, with particular emphasis on their phytochemical diversity, environmental factors, and biological properties. Available data indicate that ferns are characterized by high qualitative and quantitative variability in flavonoid compounds. In various taxa, both simple flavones and flavonols, as well as their glycosides, proanthocyanidins, biflavonoids, and other phenolic derivatives have been identified. The accumulation of flavonoids in fern tissues depends on many environmental factors, such as light conditions, soil moisture, altitude, salinity, and UV radiation. Among the biological properties, the antioxidant activity of flavonoid-rich extracts and fractions is the best documented, while data on anticancer and antimicrobial activity, although promising, still require further research. The main limitations of the current state of knowledge include the heterogeneity of research methods, insufficient structural characterization of the active compounds, and a lack of in vivo studies and toxicological analyses.