[2] The species extend from the foothills of the Hengduan Mountains, where they form dominant stands of trees, through Indochina and the Malayan Archipelago, crossing Wallace's Line and reaching Papua.

[3] These trees are intolerant of seasonal droughts, not being found on the Lesser Sunda Islands, despite their ability to cross numerous water barriers to reach Papua.

[4][5] The North American tanoak or tanbark oak (Notholithocarpus densiflorus) was previously included in this genus but recent evidence indicates the similarities in flower and fruit morphology are due to convergent evolution.

of species in Camus' treatment): Castanicarpus (1); Corylopasania (2); Cryptostylis (1); Cyclobalanus (58); Cyrtobalanus (1); Eulithocarpus (11); Gymnobalanus (10); Liebmannia (3); Oerstedia (1); Pachybalanus (14); Pasania (209); Pseudosynaedrys (9); Synaedrys (15); indeterminate (12).

An exhaustive study of the inflorescence and fruits of 73 species from eight of Camus' subgenera found that important development and evolutionary characters distinguish the major groups in the genus and indicate differences among the genera of the family.