The collection consists of four groups, namely: a collection of manuscripts, [late 16 cent.]-[18 cent.], containing transcripts of Welsh poems and prose items being largely volumes of Glamorgan origin acquired by Edward Williams (Iolo Morganwg) and forming part of his library (NLW MSS 13061-86); miscellaneous notebooks, papers, etc., of Iolo Morganwg, bound into eighty volumes (NLW MSS 13087-162); miscellaneous papers and correspondence of Cymdeithas Cymreigyddion y Fenni ('The Abergavenny Cymreigyddion Society'), 1836-1848 (NLW MSS 13182-5E); and miscellaneous other manuscripts (NLW MSS 13163-81). The second group (NLW MSS 13087-162) contains a mass of notes on matters of Welsh literary, historical, and antiquarian interest, transcripts of historical documents, transcripts of Welsh poems, series of triads, and prose compositions, and copies of original poems, essays, and reports by Iolo himself, etc. Amongst the historical and antiquarian material are items which Welsh scholars do not accept as authentic but which they regard as being spurious and the fruit of Iolo Morganwg's inventive mind. The literary material too has its quota of items regarded as forgeries, including Welsh poems written by Iolo Morganwg in imitation of the style of earlier poets and attributed by him to such poets. The accounts given of the contents of the manuscripts which belong to this second group are intended to be descriptive and not analytical and, except in the case of major 'forgeries' such as the chronicle 'Brut Aberpergwm' and the poems attributed to Dafydd ap Gwilym, and a few other items such as letters attributed to the Rev. Edward Gamage, no attempt is made to include comments on the authenticity or spuriousness of the items noted. Many of the Welsh items contained in the volumes belonging to the second group were published with English translations and notes in Iolo Manuscripts: A Selection of Ancient Welsh Manuscripts in Prose and Verse from the Collection made by the late Edward Williams, ed. by Taliesin Williams (Llandovery, 1848) and Barddas; or, a Collection of Original Documents illustrative of the Theology, Wisdom, and Usages of the Bardo-Druidic System of the Isle of Britain, ed. by John Williams (Ab Ithel), 2 vols (Llandovery and London, 1862-1874).