Vast areas of the Iberian Peninsula are covered by monocultures of the exotic tree Eucalyptus globulus. Given that (1) leaf litter produced in the riparian areas is the main energy source for small streams, and (2) trees differ in their nutrient content, chemical defenses, and physical attributes, eucalypt plantations have the potential to affect the biology of streams. Research teams from the University of Coimbra and the University of the Basque Country have been addressing the potential effects of eucalypt plantations at several levels of study. Here we review the main conclusions of these investigations.Eucalypt plantations produced less litter than some deciduous forests. However, there were marked differences in timing of litterfall: litter production peaked during autumn in deciduous forests, whereas in the eucalypt forests it tended to peak in summer and to be more evenly distributed throughout the year. Despite these differences, the average standing stock of organic matter was higher in the eucalypt than in the deciduous forest. This may be attributed to (1) the occurrence of spates or heavy rain in autumn, the period of maximum litter fall in deciduous forests, and (2) bark accumulation in eucalypt forests. Because of differences in leaf composition, the nutrient input in eucalypt forests seems to be lower than in deciduous forests. The rate of decomposition of eucalypt leaves was strongly dependent on nutrients in the water: in nutrient-poor waters it was slower than that of most other leaf species, whereas in nutrient-rich waters it can be as fast as alder – a fast-decaying species.The biomass and cumulative diversity of aquatic hyphomycetes colonizing leaves did not differ between eucalypt and other native leaf species, but fungal sporulation generally peaked 2 weeks later on eucalypt leaves. This lag disappeared when lipids (but not polyphenolics) were chemically removed from eucalypt leaves. Similarly, addition of eucalypt oils to culture media retarded or suppressed fungal growth. Streams bordered by Eucalyptus had lower diversity of fungal spores (but similar spore densities) in Portugal; less consistent patterns were found in similar experiments in Spain.Eucalyptus leaves proved to be poor food for shredders. Under laboratory conditions leaves of Eucalyptus ranked low in food selection experiments using native shredders. The same shredders failed to grow and died when fed exclusively eucalypt leaves. The removal of oils from eucalypt leaves resulted in increased feeding rates, whereas the transfer of oils to alder leaves resulted in decreased feeding rates.The effect of eucalypt plantations on stream invertebrate communities is not very consistent. In nutrient-poor waters, fewer invertebrates colonized eucalypt than alder leaves, but this effect was mitigated after a microbial conditioning period in nutrient-rich waters. Portuguese streams bordered by Eucalyptus had lower numbers of invertebrates than streams surrounded by deciduous forests. In Spanish streams differences were less marked and nonexistent when looking at the composition of the communities, which change more from year to year than from site to site. Most of the eucalypt streams studied in Portugal and Spain dried up in summer, a fact that might reflect an increase in soil hydrophobity produced by Eucalyptus plantations.The very short planting-to-harvest period of eucalypt plantations results in additional impacts, such as soil loss, siltation of streams, or reduced amounts of woody debris in stream channels, which affects their capacity to retain leaf-litter, as well as the availability of habitat for invertebrates and fish. The studies by the Portuguese and Spanish research teams confirm the importance of maintaining riparian buffer strips to reduce human impact on streams and rivers.