information:classification, structural:expansion
Species and hybrids

Over 700 species of Eucalyptus are known. Some have diverged from the mainstream of the genus to the extent that they are quite isolated genetically and are able to be recognised by only a few relatively invariant characteristics. Most, however, may be regarded as belonging to large or small groups of related species, which are often in geographical contact with each other and between which gene exchange still occurs. In these situations, many species appear to grade into one another, and intermediate forms are common. In other words, some species are relatively fixed genetically, as expressed in their morphology, while others have not diverged completely from their nearest relatives.

Hybrid individuals have not always been recognised as such on first collection and some have been named as new species, such as E. chrysantha (E. preissiana × E. sepulcralis) and E. "rivalis" (E. marginata × E. megacarpa). Hybrid combinations are not particularly common in the field, but some other published species frequently seen in Australia have been suggested to be hybrid combinations. For example, Eucalyptus × erythrandra is believed to be E. angulosa × E. teraptera and due to its wide distribution is often referred to in texts.

Renantherin, a phenolic compound present in the leaves of some Eucalyptus species, allows chemotaxonomic discrimination in the sections renantheroideae and renantherae and the ratio of the amount of leucoanthocyanins varies considerably in certain species.