Eucalypts produce very different leaves as a seedling or sapling (juvenile) or coppice growth, compared to those in a mature adult crown. One, therefore, must be wary of the nature of the leafy crown. A recently developed canopy may be the regrowth following complete loss of foliage due to fire. It is not hard to see whether an area has been recently burnt, particularly from the loss of understorey and the blackened remains of the vegetation. The first generation of leaves is likely to resemble the juvenile growth. Differences between species are often more evident in the juvenile growth compared with the adult leaves.

Adult leaf shape is not much use in identification as most species have lanceolate or falcate (curved) leaves. Leaf shape is a character of low reliability. Leaf size is useful if all the leaves in an adult crown are unusually small or large, as remarked above.

Most eucalypt species have adult leaves that are more or less the same colour on both sides. But if an adult leaf is distinctly discolorous (the upper face is darker and greener than the lower) this is a fairly powerful tool in the discrimination of species. The discoloured appearance of the leaf is a factor of internal structure, as the green photosynthetic tissue (cells with chlorophyll-bearing chloroplasts) is near the upper surface of the leaf and is lacking towards the lower surface in this type of leaf. The discoloured appearance is maintained on fallen dead leaves although somewhat faded, and the assessment can reliably be made before sampling the living crown. Juvenile leaves in all species are usually slightly to distinctly discoloured, so care must be taken in assessment of colouration.

Another protected character is the leaf venation and this can be characteristic of certain groups, e.g. the bloodwoods have many parallel side veins at a wide angle in a regularly pinnate (feathery) pattern. Other species have generally fewer side veins at more acute angles, the extreme being the snow gums (E. pauciflora) and black sally (E. stellulata) which have side veins more or less parallel to the midrib. While the angle of the side veins is highly diagnostic for the wide-angled and for the parallel-veined species, it is of little value for angle states in between.

The midrib of a leaf is the primary vein, the side veins are the secondary veins. When these are the only veins apparently present or visible, there is no reticulation, a strong character in assessing leaves for identification. Tertiary veining links the side veins and forms a reticulum. Some species have quaternary veining and the reticulum is consequently very fine. There is no absolute distinction between these categories and we use the terms no visible reticulation, sparse reticulation, moderate reticulation, dense and very dense reticulation to describe them.

Eucalypts are notable for the oil glands in the leaves. In a dried specimen the glands can only be seen with reflected light and appear as black dots on the undifferentiated surface. But if a fresh leaf is held up towards the sun and inspected with oblique light through the leaf, the glands will be seen as white or yellowish or green structures, obviously within the tissue of the leaf. This inspection should always be done by holding the lower leaf face towards the sun. This is to ensure comparability between specimens.

Many species will show quite different patterns between top-side or under-side viewing. Because the leaves turn on their stalks and hang down in the crown, some experience is needed to determine which the upper and lower faces are. This is helped if the petiole is flattened on the upper surface, as it is in many species. Difficulty will be experienced in other species in determining the upper and lower surfaces of a leaf if the leaf stalk is slender and not flattened. In these instances both sides should be examined and the image with clearer reticulation and glands assessed. This will indicate that the upper, not the lower, side is presented towards the eye. Then comparable assessment can be made.

The oil glands will be positioned either at the intersections of the veinlets, where they will appear as more or less irregular blobs, or within the un-veined areas (areoles), where they appear round or irregular, or as a mixture of both. In some species the oil glands are obscure or completely absent. The first two categories are strong aids to identification as related species will always have similar patterns.

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