Ferns are by far the largest division of vascular cryptogams, and, for purposes of publication of Flora Malesiana, they will have to be subdivided in a series of parts. This necessarily raises the problem of taxonomic subdivision of ferns into families or other major taxonomic units, a matter on which there is at present no general agreement among pteridologists, except that the more primitive families are generally recognized as distinct groups. As regards the majority of ferns therefore no formal arrangement into families will be attempted. There is however a fairly general agreement that certain groups of genera (e.g. the Davallia group) are natural groups. It is the intention to arrange the genera of ferns in such groups, and to publish the group-revisions in such sequence as may be most practicable. The author of the revision of any particular group must naturally decide the limits he will recognize for the group; and as the whole work will be by more than one author, some differences of opinion as to the delimitations of groups are probably unavoidable. It is believed that such differences of opinion will not be great, and if they occur their existence will be indicated by cross-references.