The ‘Annals of Ulster’ have long been recognised as one of the most important sets of Irish annals, yet it is surprising how little research has been published on the sources of this chronicle's tenth- and eleventh-century sections. This is perhaps because, as Aubrey Gwynn, stated, ‘That the Annals of Ulster, down to the end of the twelfth century, are derived from an ancient Book of Ard Macha is so plain that no scholar has ever questioned this conclusion.’ Such pronouncements may be correct, but they should be confirmed by proper studies, as is the case with other comparable sources. The intention of this chapter is to undertake such an analysis, focussed on identifying the sources of AU from 912 to 1100, and to consider the issue of the relationship of this part of AU to the section up to 911, which contains the last annal displaying a very close correspondence with the Clonmacnoise group. Also relevant to this issue are some items in the section from 912 to 1060 which are found with virtually identical vocabulary in AU and the Clonmacnoise group. However, these items will be discussed in chapter 4, so they will generally be excluded from the analysis in this chapter, as well as from chapters two and three on the Clonmacnoise-group texts. This approach enables a clearer comparison of these items in relation to the rest of the chronicles in which they survive, in terms of their style and the evidence they provide for locating chronicles' sources. A list of these items can be found in appendix 2.