Compared with the closely related Acrocephalus species, tree warblers have squarer tails and broader bill-bases.

Considerable evidence, much of it summarised in Parkin et al. (2004),[3] suggests that the genus Hippolais is paraphyletic with respect to Acrocephalus.

DNA studies, e.g. Leisler et al. (1997),[4] interpreted by George Sangster in 1997,[5] indicated that the olivaceous and booted/Sykes's warbler grouping (the subgenus Iduna) are more closely related to Acrocephalus species than they are to icterine and melodious warblers and as a result the Dutch Committee on Avian Systematics (CSNA) has moved these four species into Acrocephalus.

[6] A subsequent review by the British Ornithologists' Union Records Committee retained the genus Hippolais, for all eight species, but in agreement with Sangster, acknowledged that they fell into two groups.

Kenneth Williamson[10] and Hadoram Shirihai,[11] in discussing the identification of Hippolais and Acrocephalus warblers stressed the similarities between species in the two genera.